6LU8s, anyone?

RE: 6LU8s, anyone?, posted on May 23, 2020 at 21:13:46
airheadair Audiophile Posts: 393 Location: California Joined: October 18, 2010	hmmm.... I think I have a spare set or two. I'll check Do you have any 6LF6's?

RE: 6LU8s, anyone?, posted on May 24, 2020 at 07:46:01
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	I sold all my 6LF6s years ago, to Jim McShane I think.

RE: 6LU8s, anyone?, posted on May 25, 2020 at 20:25:49

airheadair
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It seems that the OTL3 uses two of these 6LU8's per channel. I think I have four spare already, but it's too hot to go up into the attic to check. If no one else wants these, I am more than happy to pay shipping to take them.

As I have probably said before, nothing I tried when I first got my speaker could drive them well, except for the Futtermans. I'm not at all competent with electronics, and now that Roger Modjeski is no longer here, I don't know what I will do when they need retubing again. They do run quite cool,
so maybe they will in fact outlast me.

Lew, can you email me some details?

RE: 6LU8s, anyone?, posted on May 26, 2020 at 08:55:45

Lew
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I have had no interest in my 6LU8 vacuum tubes. I have several, they are all brand new, the brand is RCA. I will leave this post here. If anyone, especially those who use Futterman amplifiers, sees this notice and becomes interested, I will be happy to ship the tubes to you for the mere cost of the shipping.

Please send an email via this website if you're interested.

RE: 6LU8s, anyone?, posted on May 27, 2020 at 13:52:24

Lew
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Location: Bethesda, Maryland
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What details? They are all RCA. They are all NOS. There are at least 4. It is indeed sad that Roger Modjeski is no longer around, more for his sake than for ours. But re-tubing should not require his skillset, either. I ran my first pair of Futterman amps, built for me by Julius in 1979, for at least 10 years before I even thought of replacing the original tubes. They may have been worn, but I didn't hear a problem.

RE: 6LU8s, anyone?, posted on May 27, 2020 at 18:01:19
airheadair Audiophile Posts: 393 Location: California Joined: October 18, 2010	I was just wondering about how to arrange for shipping and payment. Probably best for you to write me an email. Thanks for this offer. I've had my OTL3's retuned only once so far...over 35 years, They run comparatively cool. The instructions on how to do it seem very complicated to me.

OT question?, posted on May 28, 2020 at 11:26:01
AJ Audiophile Posts: 532 Joined: February 24, 2001	Lew, Did you ever get your Bev amps settled- if so what was the issue?

RE: 6LU8s, anyone?, posted on May 28, 2020 at 19:15:01
airheadair Audiophile Posts: 393 Location: California Joined: October 18, 2010	Sorry Lew, I can't see how to send you an email via this website. Is there a wa y to do that directly?

RE: 6LU8s, anyone?, posted on May 29, 2020 at 19:54:42
airheadair Audiophile Posts: 393 Location: California Joined: October 18, 2010	Dear Lew, Again, I don't know how to email you directly via this Forum. It requires your email, which I do not know. Best,....

RE: 6LU8s, anyone?, posted on May 31, 2020 at 07:43:52
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	I sent or tried to send an email to you yesterday using your information on this forum. Did you receive the email? Meanwhile, you could do the same. I believe my email address is available if you click on my name.

RE: 6LU8s, anyone?, posted on May 31, 2020 at 07:55:01
airheadair Audiophile Posts: 393 Location: California Joined: October 18, 2010	Yes, thank you. I received that email and answered it, and now have also replied via the forum. Thanks again.

RE: OT question?, posted on June 2, 2020 at 18:47:50

Lew
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I will let you know in the next few days. I have replaced all the power supply filter caps and the current equalizing resistors used in the power supply, most of which were toast. I have also replaced all four power tubes with new ones. The app is working fine on the bench with no load on it. Tomorrow I will attach it to the speaker and see what happens. Since I could find no evidence that the speaker panels per se are gone bad, I can only assume one or more of the old power tubes was arcing. I have no way of testing for that on my bench top, because they all behaved on the bench top.

RE: 6LU8s, anyone?, posted on June 3, 2020 at 08:50:00
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	So sorry for the delay in responding to your emails. I will send you an estimated cost of shipping today. Then we can get the show on the road.

Completely different problem now, posted on June 4, 2020 at 08:27:19

Lew
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With all new caps, resistors, and output tubes, I now have a new problem: When I re-attached the amps to the speakers, and with no input signal, after turn-on I now hear about 20 seconds of silence, then about 10-20 seconds of white noise, then about 20-30 seconds of low level buzz, then white noise. At that point, I shut down the power.

Because of the time delay after turn-on, during which there is silence, I am guessing that one of the NOS power tubes is simply noisy. To test that, I will remove and replace them with four more NOS tubes. (I have a stash.) I suppose the good news is that the repetitive clanging noise that I was hearing originally, followed by the fuse blowing, is gone, and the fuse does not blow. Also, I now sense that the original metallic clanging noise was coming from the amplifier not the speaker. The hum and white noise now are coming from the speaker for sure.

Sounds like an oscillation., posted on June 4, 2020 at 10:27:49
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	Have you looked at the output on a 'scope?

Can't, posted on June 4, 2020 at 11:21:01

Lew
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As you know, there are 3200V lurking at the outputs of this amplifier. Consequently, when I have it on my benchtop, I am running it unloaded. It exhibits no abnormalities in that mode. DC voltages are all stable and as expected. To go anywhere near the outputs with any of my oscilloscope probes would be not wise, as I am sure you know. Do you know any tricks whereby I could detect oscillations without a 'scope? I do use a 6000V probe when I measure DCV with my Fluke. I was thinking I might use that probe and look at ACV magnitude only, on the benchtop. Was thinking of making a dummy speaker load using 3000pF HV capacitors; the speaker measures as two 3000pF capacitors in series, with the screen as center tap.

RE: Sounds like an oscillation., posted on June 4, 2020 at 11:45:40

airheadair
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Posts: 393
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This reminds me again of my problem with the Patricia Barber SACD "A Distortion of Love." You may recall that with that SACD, and no others, there is background white noise which is not noticeable at low listening levels, but which becomes quite intrusive as soon as I turn up the volume to what I might call "high moderate" level. This occurs on every cut, but only as the music is playing. I have tried two SACD players, and the result is the same. I hear this noise only when playing through my NYAL amp fed to the Stax speakers, not when listening through headphones and my headphone amp. The sound level on that SACD is quite low, and I have to turn up the volume control on the preamp quite a bit more than for other SACD's, which may have something to do with the problem. I have read in a few places that some SACD's have a lot of high frequency junk that can excite oscillations in some amplifiers, so I eventually purchased some ferrite beads (chokes?) which I clamped around the output from the SACD player. This had no effect.

I don't regard this as that serious an issue, because the problem only occurs with one SACD,
and I have a FLAC version which sounds pretty good. Still, I'd like to know what is going on.
I think Ralph said that the problem might indeed be with the very high bandwidth of the amplifiers,
excited by junk on the SACD. I wonder if there is a way to measure that with a scope (I don't have one).

Anyway, just wondering.

Some SACD players...., posted on June 4, 2020 at 12:58:14

Lew
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like my Ayre C5Xe-mp, do not use a low pass filter at the output so as to eliminate a noise that is inherent to CD or SACD, which is at around 250kHz. Perhaps yours is another one of such players. With my Ayre, I could not use it at all with the Beveridge amplifiers, as they would inevitably be driven into oscillation by that high frequency. I had to use a Sony CDP instead with the Bev system. I'd like to know what Ralph thinks. I later discovered that one of the Bev amps, the one that would oscillate with the Ayre, had a cold solder joint on one of its coupling capacitors. When I found and repaired that problem, it became completely stable, but I've never tried it again with the Ayre.

If you're gonna play around with that stuff, posted on June 4, 2020 at 13:16:14

Ralph
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Then its a good idea to have one of these.

high voltage scope probe

RE: A 'scope would be handy to sort that sort of thing out, posted on June 4, 2020 at 13:22:57

Ralph
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Some amps (OTLs and a good number of solid state amps) really are not all that stable, and a bit of noise at edge of their phase margin could easily be enough to send them over the brink.

FWIW, this is one of the reasons OTLs were considered so unreliable- since they tend to have a high output impedance, to get it lower feedback is applied. They have such bandwidth that if the feedback loop doesn't observe the phase margins (IOW if the feedback loop is poorly designed) the amp could easily oscillate.

Hum, buzz and a really loud white noise sound are all signs of oscillation. You really don't want to mess with this- it can blow tweeters and power tubes out really quickly!

Since this only happens with the SACD, its a good sign that the SACD player is emitting some ultrasonic nastiness, which isn't a good thing. But at the same time it also suggests that the amplifier is a bit on the unstable side too. It would be interesting to put that SACD player on a scope with that disc and see what's really there.

Thanks and a further thought.., posted on June 5, 2020 at 08:04:38

Lew
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Thanks, Ralph. Is that probe specifically and only for "Keysight" oscilloscopes, as the data sheet suggests? I currently own a Hitachi, a Sencore, and a Tektronix. I just bought the Tektronix, and I would hate to damage it. And is that thing really insulated well enough for 3200V?

Your idea of oscillation sounds better and better to me, the more I think about it, because in the process of eliminating causes of the original mode of malfunction, which was nothing like what I have going on now, I de-soldered several parts in order to test their integrity out of circuit. Then I re-soldered them. Perhaps one of the new joints I made is faulty. That's the first thing I will look for when I get the amp back on my bench. When I had the old problem of oscillation, 3-4 years ago, which was due to a bad solder joint, I could pick up the oscillation on my scope without even touching the probe to the circuit traces. All I had to do was to hold the probe about 6-8 inches away from the source of the problem, and the scope would go crazy with VHF. Maybe the same condition will apply here, if I do have an oscillation. I'll use the Hitachi, since that is the least valuable.

EDIT. I just looked on eBay for "high voltage probe for tektronix", and I found several up to 15kV, for between $200-$300. I should have thought of this. Thanks, again.

RE: Thanks and a further thought.., posted on June 5, 2020 at 08:34:57

Ralph
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I admit that I didn't look that closely- I just googled 'high voltage oscilloscope probe' and that was one of the first that showed up.

Another way you could look for this sort of thing would be to wind some coils on a pencil and then remove the pencil. Tie each end to your scope probe and then hold it near the suspected area of oscillation (RF energy).

IMO if the amp is really oscillating then the feedback network needs to be addressed. A Pi filter network could be installed that cuts off feedback above a certain frequency. Might be a bit of a hatchet job but you really don't want an amplifier to oscillate- Bad Things happen.

RE: Thanks and a further thought.., posted on June 5, 2020 at 13:19:38

Lew
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Location: Bethesda, Maryland
Joined: December 11, 2000

The amps do not oscillate when in proper operating condition, but they are always a threat to oscillate, because you have a low voltage solid-state front end feeding a very high voltage tube output stage. So, everything has to be copacetic. I am not about to put in any sort of output filter, in any case. That's way above my skill-set. But Harold Beveridge has a low pass filter built in to the input and driver stages. So very hf stuff that might get into the amplifier from the preamp and cause oscillation, does not get there. Like I said, first it remains to be seen that the amp IS oscillating. Then, if it is oscillating, the most likely cause is a bad solder joint in the high voltage section or perhaps a bad NOS power tube or etc. (As you know better than I, finding the cause can be difficult.)

That's what I did last time, made a coil of wire and attached it to my scope probe and just held it near to the HV section of the amplifier without touching it. Oscillation was easy to demonstrate on the scope. I did find some HV probes for Tektronix, at around $1000, for 5kV.

RE: Completely different problem now, posted on June 6, 2020 at 16:00:50

AJ
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Posts: 532
Joined: February 24, 2001

I might of pointed out int he other forum that I suspected that you had a bad current hogging tube to begin with that was taking loading down the B+ and secondarily impacted the cap.. I would also check my diodes . You ruled out your panels ( less likely and the transformer)

New caps etc won't hurt - but you will not know the original problem.

If you don't want to assemble a fancy load -- three lengths of silicone test probe wire with banana plugs will get you back to the panels easy enough and temporary table with your amp on it for observation.

If your other amps tubes work.. use those and go from there-- that's what I would do at this point-- Good Luck!

Oscillation, posted on June 6, 2020 at 18:01:05

Lew
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Hi Ralph, I just sent you a private email. I fired up the amp on my benchtop with no input and no load. Using my Fluke 87 meter, I detect an AC voltage on the banana jack that feeds the mylar diaphragm, when the amp is connected to a speaker. The frequency of that voltage is 150kHz. So I am assuming you were correct; the white noise I heard is due to oscillation. Now comes the process of re-doing everything I did last week in the process of fixing the original damage which was evidently due to power tube arcing. Eventually I should find a bad solder joint or something like that that is causing the instability. Any comments or suggestions are always appreciated. I am thinking I may not need to use a scope at all; I'll just backtrack step by step and check at every step for the presence of that very high frequency AC signal at the output, using the Fluke meter. Thank you so much for mentioning the idea of oscillation, which I should have considered but didn't.

RE: Oscillation, posted on June 8, 2020 at 11:29:44
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	What does it have for stopping resistors at the grid of the tube you replaced?

RE: Oscillation, posted on June 8, 2020 at 11:54:30

Lew
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Location: Bethesda, Maryland
Joined: December 11, 2000

I replaced all 4 of the power tubes, not just one, because I had no way to know which one of the 4 was arcing. Furthermore, the tubes I replaced were "old", in terms of use. This amp appears to use something like the Futterman "totem pole" topology with two tubes on one side and two tubes on the other side of the PP output. Each of the two tubes, one per side, that receives the signal from the input stage at its grid has a 1K ohm stopper, so far as I recall. (There is no sign of the 150kHz ACV on the stopper resistors, either side.) This is also per the original Beveridge schematic, which I have here (but not in front of me at the moment). There are also 7.5M ohms feedback resistors between the front end and the output stage, which is where there can be trouble with oscillation. However, the 150kHz ACV is not seen on the feedback resistors, either.

a higher value of grid stop might be just the thing., posted on June 8, 2020 at 12:33:15
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	What power tube are you using? The grid stop can be seen as a filter network in conjunction with the grid capacitance. IOW, you can set the value to see if it sorts out the oscillation at the frequency of trouble. This isn't the only way to go about it, but it is one way that oscillations can occur!

36KD6, posted on June 8, 2020 at 16:42:40
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	nt

RE: 36KD6, posted on June 9, 2020 at 11:09:35
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	So with that tube and 1K stopping resistor, the cutoff is about 4MHz if I got this right... Is there any current in the grid circuit? If not, I would set that resistor value to about 27K and see how that works out.

RE: Oscillation, posted on June 9, 2020 at 13:58:48

AJ
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Posts: 532
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If it were me I may not wish to change the grid resistors.

This thing worked for years with out a grid resistor change. I'd very carefully examine the sockets and the PCB- clean the board and sockets with hogs hair brush and flux remover .. follow up with alcohol.

I would swap the tubes from the other amp I know I am not having trouble with. Tubes can be a funny thing in this circuit.

I would swap the amp to the other panels after that to observe if no change.

Then if no further ahead - I would start looking at leakage and gain on those input driver transistors.

What do your PS look like , good amp vs. bad amp loaded to the same panel?

You have the advantage of a good amp for comparison in whatever you decide to do. You may need to truly eliminate the panel... by trying the good amp on each of the panels & observing.

RE: 36KD6, posted on June 9, 2020 at 16:20:54
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	If I have to go down that road, I will have to obtain some proper type resistors. I will do that just in case. I'd rather not deviate from the values shown on the schematic, unless I have to. (This may sound strange, coming from me, I admit.)

RE: Oscillation, posted on June 9, 2020 at 16:52:06

Lew
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Location: Bethesda, Maryland
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I don't disagree with you about not changing the grid resistors, at least not yet, as I also responded to Ralph below.

Here is some more info that I communicated to Ralph privately: "I found that one of the wires to the anode cap of one tube was worn from flexing where it solders into the PCB; it broke off as I was trying to insert a replacement tube. It has been my past experience that worn solder joints or wires that have been flexed one time too many do give rise to oscillations in these amplifiers. I stripped some insulation to access fresh wire and re-soldered it and also replaced all 4 tubes. (The tubes I'd been using already were new ones, but it has been my past experience that some tube types are just more prone to oscillate than others. RCA brand seem to be the most trouble-free, and I replaced a mixture of new Sylvania and GE with all new RCA, now.) This leads to better measurements as regards oscillation, but I am not sure I won't have an audible problem when I re-connect the amp to the speaker. On each of the stator outputs I now see very low ACV, about 5-10 mV where DCV = 1530V. My Fluke says that this ACV does have a high frequency (~170kHz), however. On the screen output, I see zero VDC, as expected, and about 60mV of AC. I have never measured ACV on any of these nodes before, so I do not know whether this is normal or not. The frequency of the ACV on the screen output is around 30 kHz. Previously (before the empirical tube swap to RCA), I also was seeing about the same 60mVAC on the screen output,as now, but with a frequency of 150kHz or so, which I took to be the manifestation of an oscillation. The AC voltage and its high frequency can be detected coming off the 7.5M resistor that goes to the cathode of the top tube in one totem pole and then connects on to the screen output. I am wondering whether to just give it a try at this point on the notion that maybe the AC will be inaudible and was always there.

Yes, it would be helpful to know the ACV values seen in a functional amplifier, on my other speaker. I have been trying to avoid the labor involved in separating that amp from its speaker, due to back problems, but maybe that has to be done."

AJ, I tried to send email to you privately but you are not receiving via Audio Asylum. Which is fine. However, you can contact me via the Asylum. I greatly value the advice of anyone else who has worked on Beveridge amplifiers in the past. These amps have been thoroughly gone through not only by me but also by Bill Thalmann of Music Technologies in Springfield, VA. Pretty much the only original parts remaining are the coupling capacitors (all 3), those 2.2M resistors (Caddock, so originals may have been replaced by someone else.), the anode caps and wires to the anode caps. Bill installed 4 new tube sockets back right after I bought the system. I've cleaned all soldered areas with acetone on cotton swabs. Power transformers are dated 1978, so are probably OEM.

RE: 36KD6, posted on June 9, 2020 at 19:53:32
airheadair Audiophile Posts: 393 Location: California Joined: October 18, 2010	How close are those tubes to 6LF6's?

36V filaments for 36KD6, posted on June 10, 2020 at 09:26:50
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	Otherwise, I guess they are in the same family. 6LF6s are far more expensive and rare, however, as you may know.

Huh??, posted on June 10, 2020 at 10:55:53
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	Where is Lew and what have you done with him? Seriously, after all the mods you've done, this one is pretty easy and might just solve the issue.

The Beveridge amps are so fussy as is...., posted on June 11, 2020 at 07:23:37

Lew
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Location: Bethesda, Maryland
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that I have stuck to every single element of the original schematic, which I was lucky to have obtained from Roger Modjeski a couple of years ago. There are no modifications, except new parts and better capacitors in some cases. I finally got out one of my oscilloscopes and did the dangling coil of wire trick to see if I can detect oscillation. By this criterion there is no oscillation, now. (I fixed one more iffy looking solder joint before that.) So, I am thinking maybe the very low voltage, high frequency AC I am picking up with my Fluke frequency counter is of such low magnitude that it won't be audible. Also, it may always have been there, since I have never used the Fluke in this way before.

What I might do is to move the 1K grid stop closer to the grids they serve. Beveridge built them sitting in space about half way from the input board to the grid pin of the output tube, about 1" to 1.5" from the grid of the driven output tube. I have seen many photos of other Beveridge direct-drive amplifiers; they were all built like that. Even the schematic depicts a distance between the 1K and the output tube grid. (But no distance is stipulated in the detailed schematic.) In this one of my two amplifiers, they are perhaps closer than normal to the input side, which leaves maybe 2" to the output tube. Back when Bill Thalmann did work on these, right after I bought them, he added some ferrite beads between the body of the 1K resistor and the input board, to suppress oscillation. Bill did the same on the 7.5M resistor lead. I haven't messed with that.

I think moving the stopping resistors closer to the grids is a good idea. nt, posted on June 11, 2020 at 08:54:08
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	-

Done., posted on June 11, 2020 at 13:53:32
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	We'll see what that does. The bodies of the 1K resistors are now within an inch of the grids. Not "close enough" normally, but close for any Beveridge amplifier I've seen.

Did any of your changes and repairs sort it out? nt, posted on June 16, 2020 at 10:32:45
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	-

RE: Did any of your changes and repairs sort it out? nt, posted on June 17, 2020 at 08:42:31

Lew
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Location: Bethesda, Maryland
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I think yes.
In the course of making repairs after the fuse blew for the second time, which event also took out some of the filter capacitors and some of the current sharing resistors in the power supply output, I substituted carbon comp resistors with metal film resistors. Because the latter are physically larger than the originals, I more conveniently mounted them on the backside of the circuitboard. As it turns out, that was a big no-no, and was causing the noise. Whether it can be called oscillation or just noise I don't know. An EE tells me that it doesn't meet some of the definition of an oscillation, because there are many many mixed frequencies going on at the same time. Anyway, when I remounted those metal film resistors on the other side of the circuitboard, the noise is definitely reduced by at least 90%. I also found some high-voltage wiring that was weakening at its solder joints, because it's very thin gauge and flexing was weakening it. I resoldered those joints, and that helped too.But those metal film resistors were definitely the major culprit.

Good!, posted on June 17, 2020 at 09:36:21
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	If moving the resistors like that did it, it is without any doubt an oscillation. What happens is the oscillation frequency gets rectified in other parts of the amplifier and you get all kinds of intermodulations that basically sound like noise. That EE you spoke to does not have a lot of hands-on experience with things like this.

Premature judgement, posted on June 18, 2020 at 08:47:27

Lew
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So, when I reported a 90% improvement, I hadn't yet moved all of the current sharing resistors to the safe side of the PCB. Three out of 8 were still on the sensitive side of the board. I finished the job yesterday and was hoping the result would be a small further lessening of the noise, which I am measuring by AC voltage on various parts. To my chagrin, that is not the case. In fact, after moving those last 3 resistors, the measurements are not quite as good as they were yesterday. I changed nothing else. It's not a complete regression, but not an improvement either. And for all I know, the amplifier would "work" if I connect it back to the speaker. It's getting to the point where I have to just try it.

I have one more trick to try first. Years ago when he was also trying to eliminate oscillation after I first purchased the system, Bill Thalmann put some ferrite beads on the leads of the 1K grid stop resistors, on the side nearest to the low voltage input stage. When I moved the bodies of the 1K resistors closer to the grids of the output tubes they serve, I also moved those ferrite beads closer to the HV output board. I wonder whether THAT is now contributing to the random noise. I will move them back toward the input board and see what happens.

RE: Premature judgement, posted on June 18, 2020 at 09:30:50

Ralph
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Hm.

This really supports the idea that the layout and component choice has resulted in a circuit that is unstable and on the verge of oscillation on any given Sunday. Moving the current limiting devices should have had no effect whatsoever on the noise. But if you were having an RFI problem, this could have had a big effect- which later might not have appeared to do what it did earlier. RFI is like that- its inconsistent. An oscillation essentially produces RFI in the circuit- and as a result can act almost exactly the same, as the radiation gets everywhere in circuit.

I really would change out those stopping resistors, as 1K seems entirely inadequate. From your description (and what I've seen of them), the amp seems to have been designed around a 'by gosh and by golly' approach rather than the math actually worked out in all the areas. The 1K stops seem to me an example of that- more an acknowledgement that you need them but not actually a value that would do what they are meant to do. Its not a big deal- you've already moved them. If they don't calm the amp down a bit you can always change them back.

RE: Premature judgement, posted on June 18, 2020 at 13:42:36

Lew
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Noise went down when I moved the 1K resistors AND the ferrite beads back to where they were when this saga started. For what it's worth, now that the levels are lower, I can trace the noise to the top tube in the totem pole that feeds the screen. That tube has no direct connection to the 1K resistor, which of course supplies bias and signal to the grid of the bottom tube in that totem pole. But I can see that most remaining noise is on the 7.5M resistor that attaches from the input stage to the cathode of the top tube and then to the screen outputs. (I assume it's feedback, but I could be wrong.) Also, this is with nothing attached to the output. Could it be that loading the amp would reduce the electrical noise?

And yes, oscillation or at least noise, is a known problem with these amplifiers. If I did not love the sound of the system so much, I would not put up with it. But I do, and I am. Roger Modjeski had mods for the amplifiers, among which was using tubes in the input stage. But apart from that, I don't know what he did to them. Too bad, for us owners and for him, that he is no longer around.

I have a good schematic (bought from RAM) which I might be able to send to you, if you're interested.

I think I can give it a go on the speaker now.

Nearly any amp will be noiser with no load nt, posted on June 18, 2020 at 13:57:27
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	-

RE: Premature judgement, posted on June 18, 2020 at 14:11:19

Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000

I measured the speakers when all this started. From stators to screen the resistance is infinite. There is 1.5nF of capacitance from one stator across to the other, and as one would expect, there is about 3nF of capacitance between each stator and the screen. I bought some 6kV rated, 3nF capacitors off eBay with the idea to create a dummy load (inside an insulated box, of course). I'm a little afraid of trying that, however. Two caps in series where the stators connect at each end of the chain, and the screen attaches to the junction between the two capacitors. (On these speakers, the screen is low impedance and held at zero potential and is driven by signal voltage.) Each stator is at +/-3200V and also driven by signal voltage.

I think you should try it. Just keep an eye on the amp. nt, posted on June 19, 2020 at 08:36:34
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	-

That's a problem... (keeping an eye on the amp), posted on June 20, 2020 at 19:30:06
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	When installed, the amplifier is the the base upon which a 6-foot speaker cabinet rests. You can't see the tubes or anything else that is going on. You have to go on funny noises, either from the speaker or from the amp chassis.

Can you extend the speaker cables for test purposes? nt, posted on June 22, 2020 at 08:33:21
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	-

In the standard set-up there are no cables., posted on June 24, 2020 at 10:32:24

Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000

The amp connects directly to the speaker via HV banana plugs. The stators receive +/-3200V. Can you imagine a "safe" cable for that? I've got 20kVDC wire, but not enough of it. I remember seeing a photo where some guy in Europe inverted his Beveridge speakers, so that the amplifier sits on top. Then he removed the "bottom" part of the clam shell chassis (which becomes the "top" part when the whole speaker/amplifier is inverted). Thus he could see and access the amplifier while playing music, if he stood on a step ladder, because the speaker alone is at least 6 ft tall. I honestly don't want to be standing on a step ladder next to +/-3200V that is riding on a structurally "tippy" cabinet. I guess for test purposes, you could lie the speaker on its backside and use 20kV wire terminated in HV banana plugs on the amplifier end and HV banana jacks on the speaker end, to link the banana jacks on the amplifier, which would be sitting on the floor next to the speaker, to the relevant male banana plugs on the speaker.

You need spark plug copper wire for that , posted on June 24, 2020 at 10:39:29

Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002

ebay link

https://www.ebay.com/itm/7mm-Copper-Core-BRAIDED-CLOTH-Green

RE: In the standard set-up there are no cables., posted on June 25, 2020 at 13:15:50

AJ
Audiophile

Posts: 532
Joined: February 24, 2001

Up to 10kv test prod wire is readily available. You will be able to run 6-8' of cable between the amp and panel with banana plugs and jacks.

This is what RM detailed to me when he was building and repair amps and testing with open panels.

I run about 6' of cable to my DD amps with high voltage silicone wire in as similar way

https://canada.newark.com/pomona/6733-0/test-lead-wire-silicone-18-awg/dp/63H2513

RE: In the standard set-up there are no cables., posted on June 27, 2020 at 10:03:03

Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000

Yes, I am sure that COULD be done, but I wouldn't want to do it except in the case where I am trouble-shooting. Otherwise, I find it nice and neat and safer to have the Beveridge amplifiers serve as the base for the speaker with no external wiring. This system is in our finished basement which I like to keep only dimly lit while listening to music. As it is, the IC from the preamp to the L hand Beveridge travel across the floor between me at my listening position and the speakers. More than once I have tripped on that particular IC on occasions where I was turning the system on or off. I'd hate to do that with speaker cables carrying +/-3200VDC.

Is there a particular reason why you have chosen to use external speaker cables? I guess it may be inherent to the fact that you converted Model 3s to DD amplification.

RE: In the standard set-up there are no cables., posted on June 27, 2020 at 15:37:21
AJ Audiophile Posts: 532 Joined: February 24, 2001	I'm only suggesting this as method to troubleshoot. For my Model 3's the DD amps were never part of the structure so I use HV cable to tie the amps to the panels.

RE: In the standard set-up there are no cables., posted on June 30, 2020 at 08:02:56
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	I appreciate your input greatly.

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6LU8s, anyone?

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