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99.5.232.186
This is it... and it sound decent..
As promised....
Willie
Follow Ups:
Yes it ( my Amp) will play without the bias resistor, but it will exhibit some instability.
This would show as one channel not playing at times, or would fade out sometimes when playing at highest volumes. Bias reading between channels were also different
I guess it really didn't start showing up until I started testing it more and moving it around a lot.
I installed a 100k resistor on the grid of each V2. Now both bias meters read about 28ma on the 45's and plays at full volume.
Thanks Jim Dowdy who spotted this problem a while ago ....!
I should come here ask for .... and listen ... to you guys more often .!!
Thanks All
Willie
I would expect that 26 tube to draw heavy current and maybe self destruct without a grid resistor to ground.
That didn't happen, but they did play music !
Willie
TRY 350K - give or take 50K, grid to ground, second Type 26 !!! TRY it .
Jeff Medwin
Sounding pretty good !
Willie
Should be fine now. You are there Willie !!! Congratulations.Happy you posted on this Forum for all to see your own amp, schematic, and to comment upon !!
Jeff Medwin
Edits: 01/21/15
That plagued me after I added the second 26 ..But it appeared so randomly it had me fooled.
Now she's as solid as a rock , has plenty of gain with No pre. But with an active pre it just slams. What kills are those Hammond 125ESE's. They are such a bargain for bringing a project forward.
I'm happy with the results of this experiment,I hope others will give it a try.
Have fun
Willie
If you could post (or send me) the final schematic.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
is by using a signal generator and a scope, that is what I am learning now.
for most of my build I've relied on music input and speaker output.
Thank you for your suggestion!
Willie
Willie:
Get it as good as you can musically. Your 100K is still too much load. When it's as good as you can get it, apply the signal generator and dual-trace 'scope.Watch your thinking! You want signal replication, but NOT at the expense of any dynamics, speed, or transparency. You WANT IT ALL.
Keep going back and forth. After noting the test instruments, make necessary corrections and go right back to listening, then-- after a few weeks or so-- back to the tests, etc.
When you think it's good, call-in your NON-audiophile friends and throw a Party. They will tell you if it's good or not.
The audiophiles can come in and grasp at it after it's perfected. Until then, let them pontificate with their own stuff.
Here's a little story to encourage you-- and other good guys here: A while back, I attended VSAC in Washington State. There were only 35 or 36 exhibitors there.
It was mainly a DIY affair, but was open to us semi-commercial guys as well.
There were all kinds of DIY people present, and a contest downstairs to see who could build a SPUD amp in a day, or so-- or was it hours? Don't remember. The DIY guys did it, and it sounded OK to me. They're pretty talented!
I had my usual demo going on with the big GPA 604's in a terrible room with a huge ceiling vault, and really bad echo/acoustics. So, I dutifully placed the two speakers very close together-- in a huge room, no less, and killed all the bass I could because the room was just too big and echoey to control. No hope for my time and budget!
So, we just set it up so we liked the music-- to "H" with the @@#$%^ room!
I knew this was no show-winner, but we were having fun anyhow. I just let it play and cruised around the other places, enjoying the DIY stuff, and all the cool LP's one could purchase.
I got back, and the Hotel crew (that was on a break) was in my demo room. They had been to all the other rooms, and liked this one-- said it sounded "real" to them-- the others didn't. They loved it, and over the next few days, the whole Hotel crew was spending every off-moment in our demo room.
The DIY people and Audiophiles didn't care too much-- they said good amps, bad demo, etc. They were Nit-Picking, the normal people were enjoying music despite the room.
If you're developing equipment to listen to, you have to decide what is real.
---Dennis---
Edits: 01/20/15
Thats when you know you are on to something
Thanks, George-- good Banjos. reminds me of Southern California. I like it!
Speaking of S. Cal., you might like these CD's-- Chris Hillman and Herb Pedersen at Edward's Barn (Rounder
11661-0652-2), and Chris & Herb-- "Bakersfield Bound".
---Dennis---
The larger you make the value of the grid resistor (within the limits of the tube) the happier the stage before it will be."I generally stick to grid leak resistor values of 250K or less with the 26 as they tend to have significantly higher grid current than more modern types.
Grid leak resistor values aren't given in any 26 datasheet I have looked at - probably because the 26 was commonly used in transformer coupled circuits. "
As a separate issue, with a 100k resistor and a .1uf coupling cap you will have a -3db point of that high pass filter at 15.9Hz. The amplitude and phase will remain somewhat disturbed clear up to 159Hz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/19/15
after putting the 100K resistor in the wrong place. I'm not going to post that schematic again
Willie
That's not where you need the bias resistor, you need it after V1.
Wouldn't that be at V2 after the coupling cap? V1 has a grid choke.
Dennis suggested 350K or 250K.
Yeah, oops, after V1.
With a 0.1 uF coupling cap, Dennis' suggestion, start with about 350K Ohms, listen and measure, then TRY 250K Ohms, listen to that, makes very good sense to me. I am glad Willie is open minded enough to give it a TRY !!There may be a run on 5VCT at 3 A. wall warts, at local thrift stores, all over America now !!!
Jeff Medwin
Edits: 01/19/15
For me this amp was just an experiment into all DHT power in a cost effective way. The state that its in now is what has got me sound out of both the OPT's .
What the schematic represents is as complete q representation of what is on the circuit. That it actually makes such good music is amazing to me.
Now that it's out here on AA , offers anyone who wants to can take it from here. As for me, I'll only plan to get better OPT's..
Willie
Hello drummerwill,
Nice looking build. So how does it sound? You say modestly 'decent',but could you compare this with your other amps? Your general impressions would be nice.
I'm still trying to learn this stuff so if you don't mind, I have a few rookie question about your schematic. First, the schematic says for one channel, so I'm thinking monoblocks but your picture shows all the tubes on one chassis. Are there two or one power supplies? From the labeling I'm confused about the B1+ Lchn, B2+Lchn and B2+Rchn ??
Regards.
David
Sorry for the bad labeling on that one, this should better explain the design.
How does it sound compared to my other amps ?
Well it doesn't have the authority that my 6SN7-DC-2A3-IT-845 amp has nor the extended bandwidth of my 2A3 DC Parafeed amp. But what it does have is a very Clean sharp musicality across the bandwidth.
I mean you can hear the DHT front end driving the 45's. .... So crisp so clear. The type of reproduction of music you never want to turn off!
OK I've said too much . ..Sorry .... it sounds decent !
Willie
The correct value for the Bias meter resistor is 10 ohms !!
Willie
Thanks for the corrections drummerwill. I want to build this amp, so please indulge me to ask a few basic questions.1.)First, where should I connect the other primary line of the 5U4 filament transformer?
2.) Why are the B2+Lch (332V)and B2+Rchn (339v) voltages different and the voltage is 337v on the signal schematic?
3.) You show no values for the bypass caps. What are good starting values for the 26 and 45 tubes?
4.) There's a cap after the 24uf 630V in the PS with no specified value. Is it necessary?
5.) I would need two dual 26 supply, one for each channel, correct?
6.) Would two 5VDC 3A Wall Adapter Power Supply work for the 26 filament supplies or.. ?
7.) Two 2.5Vac transformers and associated caps and resistors, one for each channel are needed, correct?
8.) Can you direct me to a schematic for a LM335 based regulator? My searches shows the IC as a temp. sensor.
9.) I may have more questions later. I really need to build this amp!!
Thanks again for taking the time to share your work.
Regards,
David
Edits: 01/18/15
It looks unbelievably simple (good) on the signal side (not a lot of stuff in the way of the signal) to my non-DIY brain...and it works!
Observe, before you think. Think before you open your yap. Act on the basis of experience.
1) Not sure what you mean
2) Stupid record keeping mistakes. ( Shoot for approx. voltages)
3) This is not mine to give !
4) not specified.... it's all DIY
5) Yes
6) Give it a try !
7) YES
8) Google is your friend .
9) Please be careful ... and Have Fun !
Willie
Hello Willie,
Thanks for taking time to answer my questions.
1.)On the filament transformer for the 5U4, I only see one connection on the primary side.
8.) Google must not like me. The only reference to a LM335 is a temp sensor. A little help Please.
Thanks again,
David
Aww yes. must be a typo ... I think it's LM338 for a 3amp 3 term. V.R.
CT of the 5V PT not connected.
Willie
The schematic shows no bias reference for the grid of V2.
[R(g) or grid choke.]
for your suggestion. I can only tell you, I tried putting a grid resistor to ground on V2 ...it didn't work on my amp.
I can only show what has worked for me and I think the circuit shown
represents what I have now that is working.
Again Thank you for your input !
Willie
Willie--- what was the value of the grid resistor-to-ground that you tried on V2?
--Dennis--
with that I got no sound !
It sounds decent without a Grid resistor !
Willie
Hi Willie.
Try 350K, then try 250K. Let me know what happens.
---Dennis---
"Dennis I tried a 50K ......with that I got no sound !"That's a clue. I still don't know what it means but *no sound when a 50k grid resistor is in place* is a clue.
Do you have 2 channels behaving this way?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/18/15
What value resistor would be worse than no resistor?
There's no such thing as too low, at least in terms of the function of the tube itself.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What voltage is the grid of the 26 sitting at? What didn't work with the resistor?
"I tried putting a grid resistor to ground on V2 ...it didn't work on my amp."
So the amp runs and sounds good with no bias reference for the second type 26 tube?
That's really interesting.
I wonder what's going on there.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yeah Tre'
It seemed strange after others had suggested I needed a grid resistor.
But thatsit.
Willie
Maybe this addresses what some may view as a compromised PSU design?
If the grid is not bias referenced (ie; some connection back to the cathode. Normally done with a resistor or grid choke to ground so the cathode resistor can provide bias for the tube or in the case of a direct coupled circuit the reference is back through the previous stage), the tube cannot work properly if at all.Does anyone have any insight as to what's going on here?
The amplifier is reported as working and "sounds decent".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/18/15
1) Bad V2 with internal resistance between cathode filament and grid?2) Enough dirt or other substance on tube socket to act as a grid reference?
3) DC Leakage through coupling cap?
Edits: 01/18/15
otherwise I wouldn't think it would work at all.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm still pondering how V2 functions as drawn.
Who knew?
a learner how that could be the case?
Thanks in advance.
91.
Any issues with microphonics?
No problems I've heard .
Willie
Try it with rod colemans power supply
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