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Input tube suggestions?

24.201.231.29
Posted on July 11, 2016 at 13:28:37
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 Hi. Looking at that 6EM7/300B amp (Coincident), I think a different input tube with half gain of section 1 would "fit" for a 300B. Having an 1:1 interstage with a good swing (120V peak), maybe a 6DJ8 (mu=33) or a 6922 (also mu=33) could be good candidates? What do you think

 

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RE: Input tube suggestions?, posted on July 11, 2016 at 16:47:57
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 If you're after full power from the 300b I don't think the 6dj8 will give it.

The 6dj8 has a plate voltage limit of 130vdc. Depending on idle current and load line I think there's only about 50 volts peak before it hits 0 grid voltage and grid current.

The 6922 can take a lot more plate voltage.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Input tube suggestions?, posted on July 11, 2016 at 18:22:23
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 Thanks. I realized I didn't make myself clear. Just trying to match the triode 2 of 6EM7 (low rp) with another input tube instead of section 1 (high rp, gain of 64). There is too much swing (around 240V peak) right now. One possible solution is to replace the first section by another tube with lower gain. It could be also a complete different tube but it has to be low rp (<1.2k) as driver.

 

RE: Input tube suggestions?, posted on July 11, 2016 at 18:54:59
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 What about a 6sn7? It's a very linear tube.

I don't know your circuit but let's say you get a gain of 15 out of the 6sn7 input stage and a gain of 4 out of the driver stage.

That gives you a gain of 60 so 2 volts RMS IN would give you 169 volts peak at the grid of the 300b.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

The best I know, but not simple...., posted on July 12, 2016 at 02:31:39
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 I would say, having built or heard many DHT implementations, that the best first stage I know is Ale Moglia's 01A preamp stage. It's truly magical.

http://www.bartola.co.uk/valves/dht-pre-amplifier/01a-preamp-gen2/

I would equally say that the best driver stage I know is a 4P1L in filament bias with Rod Coleman's regs, and ideally a choke input filament supply.

http://www.bartola.co.uk/valves/2016/05/08/4p1l-preamp/

http://www.bartola.co.uk/valves/for-sale/gyrator-pcb/

Ale Moglia is a friend, and I've tried out his ingenious designs over the years as they got more and more sophisticated. We started off with 26 preamps over 5 years ago, then went on to 4P1Ls, and simultaneously with Ale's gyrator designs, Rod Coleman was developing his filament regs and Thomas Mayer was showing how filament bias could eliminate cathode bypass caps. Three very clever people who have between them moved the state of the art forward way beyond the simpler solutions like 6SN7s.

It's not a simple build, but it's a whole level up in sound quality if you make the decision to go for it. There's plenty of support on DIY Audio for all these new designs - long threads on 26, 4P1L with build instructions, schematics, tweaks and so on.

 

RE: Input tube suggestions?, posted on July 12, 2016 at 03:45:35
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I have been using a DIY 91A clone for my main amp for quite a few years and I have tried the 310A, 6sn7, and the ef37a as driver tubes. So far I like the sound of the ef37a the best. It's just a drop in, no design changes required that I recall.

 

6S45P/417/5842/437-nT, posted on July 12, 2016 at 08:55:41
Cleantimestream
Audiophile

Posts: 7551
Location: Kentucky
Joined: June 30, 2005
 !
The Mind has No Firewall~ U.S. Army War College.

 

RE: Input tube suggestions?, posted on July 12, 2016 at 09:22:59
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Absolutely terrible idea. Leave the amp's tube line-up INTACT !!!

You NEED the gain of the 6EM7 , mu 68 to have a lively sounding amp at normal listening and use levels. A mu of 100 would be even better sounding. ( Paralleled two matched 12BZ7 sections ).

A mu of 33 would be very ORDINARY and ho-hum sounding. Dead on its rear end.

And WHY in the WORLD would you want to add ANY 1 to 1 transformer in series with the signal, when its not needed. WHY ??

Jeff Medwin

 

RE: Input tube suggestions?, posted on July 12, 2016 at 10:32:04
Cleantimestream
Audiophile

Posts: 7551
Location: Kentucky
Joined: June 30, 2005
 How about a 5687?

Just parallel both sections
The Mind has No Firewall~ U.S. Army War College.

 

RE: Input tube suggestions?, posted on July 12, 2016 at 11:02:30
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 Well, not everyone likes iron for an interstage job...But I'm not here to argue whether or not my amp should have a transformer there. Thanks for your suggestion.

 

RE: The best I know, but not simple...., posted on July 12, 2016 at 11:03:27
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 I know these guys and have a 01A line stage. Not Ale M. version but use Rod C. regulators (filament bias) and It sounds great.

 

RE: Input tube suggestions?, posted on July 12, 2016 at 14:41:33
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 The 6EM7 is a dual triode. What is the large-triode side doing? The answer to your question depends a lot on that.

 

RE: Input tube suggestions?, posted on July 12, 2016 at 19:29:32
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016



I thought it would be better to show what I have in mind.

Keeping both sections of 6EM7, it would look like this.

Section 1: around 1mA, 200V, -2V
Section2: 42mA, 160V, -24V

300B: 60mA, 350V, -74V

Interstage transformers are designed for low rp tubes (600-900 ohm optimum. Ok from 300 to 1200 ) and from 40 to 80mA. Inductance around 31H (I over 30mA) . They can swing 85Vrms. DCR 152R/153R.

Don't know if I'll put a resistor at secondary of IT.

Power supply not designed yet (as you can see).

Maybe I forgot details but I'm not really a designer...Suggestions are welcome, thanks

 

RE: Input tube suggestions?, posted on July 12, 2016 at 20:58:35
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
It looks like you want to set up a 3 stage amp. It will have it's virtues and trade-offs. I use an amp with the same topology as you posted, though the tube lineup differs. I use 76-6AH4-xfmr-300b. The 76 is one of the best sounding IDHT of them all. The 6ah4 is chosen for it's ability to drive the xfmr, and is also a good sounding tube. The gain of this lineup is ~ 70; which is adequate. your topology is good; you might want to use better sounding tubes than the 6em7.

 

RE: Input tube suggestions?, posted on July 12, 2016 at 21:27:01
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 That's what I hoped. In this case, a 6DJ8 or similarly voltage-limited tube would be no problem, since the first stage only has to drive some 15-20vRMS to get the full 85vRMS out of the interstage.

 

RE: Input tube suggestions?, posted on July 13, 2016 at 02:56:23
Johno
Audiophile

Posts: 388
Joined: June 9, 2002
 Well I've mucked around with a very similar 3-stage cct & here's my 2c worth.

1-The low gain portion of the 6EM7 used as the IT driver sounds best with 25ma + of plate current I see you have that covered great more fulsome sounding. Quite often you see design with plate current <15ma in this configuration - no good. The 6EM7 is a robust valve & happy with AC heaters one of the virtues of IDHT.

2-Found the high gain portion of the 6EM7 used as an input valve to have way too much gain & also a bit bland sounding. Swapping input to a 6J5 requires an additional valve but LC coupled (Lundahl 1667-15ma) at 160V P-k @ 10ma to the low gain section of 6EM7 is excellent sounding. 6J5 can also be run on common 6.3V AC heaters with 6EM7. Way better sounding more character & a lower noise floor to boot. Hum at spkrs was <1mV. If tempted you can use this cct with some of the more robust 300B design made from EML like there 520B or XLS versions.

Cheers Johno.

 

RE: Input tube suggestions?, posted on July 13, 2016 at 05:44:31
jetrexpro
Audiophile

Posts: 57
Joined: April 19, 2010
 If you are into trying the 91a amp (pentode driving the 300b) Try a 6C6 pentode driver which would be a very traditional approach. A more modern approach would be the C3m in pentode. 225v on the Plate and a regulated 150v on the screen.

 

RE: Input tube suggestions?, posted on July 13, 2016 at 08:48:48
sideliner
Audiophile

Posts: 208
Location: NYC
Joined: August 22, 2013
Contributor
  Since: December 15, 2023
 On a side note, based on your desired operating point for the 300B while taking into account that the tube is cathode biased, the plate voltage will need to be at 422V if you want 350V through the tube (422V-72V = 350V). The 72 Volts are dropped at the cathode resistor (1.2k x .06A = 72V). Since you'll probably be dropping another 20 or so volts at the transformer primary, you should be designing the power supply for a B+ of around 445V

 

Here's a thought, posted on July 13, 2016 at 09:52:29
sideliner
Audiophile

Posts: 208
Location: NYC
Joined: August 22, 2013
Contributor
  Since: December 15, 2023
 The way the high mu section of the 6EM7 is set up now, it has an in-circuit gain of 51. By eliminating the cathode bypass cap the gain will drop to 25. Not only will this achieve a gain reduction by 6dB, but it should also quite nicely reduce distortion from the degenerative current feedback. You will be eliminating a capacitor that's in the signal path while getting your reduced gain along with improved distortion in the process, a nice win-win IMO.

Even if you go with an additional input tube such as the suggested 6SN7/6J5, it would be very simple to try what I've described here. It would only require a couple solder connections and at the very least it should be a good education.

 

RE: Input tube suggestions?, posted on July 13, 2016 at 11:32:46
hennfarm
Audiophile

Posts: 535
Location: Oregon
Joined: October 8, 2008
 6bl7 or 6bx7. JH

 

Ah! We're on the same page., posted on July 13, 2016 at 14:02:52
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 If you know Ale, Rod and Thomas then you're in the right company!

I've built several 26 preamps and 4P1L amps, but the Gen 2 01A blew me away when Ale brought it over and we A-B'd it with 4P1L and 26 preamps and some other stuff. The Gen 2 into 4P1L driver into 4P1L SE was a pretty perfect combination.

This thread reminded me I need to build his Gen 2 preamp, and I just got all the parts in. He has some new gyrator PCBs on his site. It's all getting very sophisticated, with Rod's regs several revisions down the line from the original. Some very clever stuff going on.

 

good suggestion, a no loss, 'learning' experiment~nT, posted on July 13, 2016 at 15:49:46
Cleantimestream
Audiophile

Posts: 7551
Location: Kentucky
Joined: June 30, 2005
 ~!
The Mind has No Firewall~ U.S. Army War College.

 

RE: Here's a thought, posted on July 13, 2016 at 16:31:11
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 Removing capacitor will reduce gain (NFB at cathode) and increase Zout.
If I have: 40k + (68+1) x 2k = 178k
Then new Z = 178k II 100k = 64k
New gain = (68 x 100k) /(100k x 178k) = 38dB
Am I right?

 

RE: Here's a thought, posted on July 13, 2016 at 20:02:34
sideliner
Audiophile

Posts: 208
Location: NYC
Joined: August 22, 2013
Contributor
  Since: December 15, 2023
 I admittedly got a little lazy and used that same operating point but on the very similar 6SL7 instead, since I could obtain the parameters for that tube much easier. Following that, I went back and had a look at the 6EM7 datasheet and for your set up as shown, the OP is at Va = 152V, Vg = -2V, ra = 50K and mu = 64. I didn't check your math, but keep in mind that the impedance looking into the cathode is in parallel with Rk and I don't think that you took that into account. In any case, I've developed an excel spreadsheet that calculates Zo, gain, etc., so with the aforementioned ra and mu values that correspond to this specific OP, the numbers become:

Zout -- bypassed = 33.3K
A -- bypassed = 42.7

Zout -- unbypassed = 64.3K
A -- unpypassed = 22.9

As you can see, the input section of the EM7 is hardly high gain in the unbypassed config. Going with say a 6SN7 that's going to yield a mu of ~17 will not make that much of a difference, but of course that will have to be a judgement call on your part. Just let us know what happens and if you happen to try both schemes it'll be all the more interesting.

 

RE: Input tube suggestions?, posted on July 13, 2016 at 23:39:41
Donald North
Manufacturer

Posts: 1296
Joined: February 8, 2001
 I would use a different dissimilar dual triode like the 6DE7. The input tube had parameters more like the 6J5.

 

RE: Input tube suggestions?, posted on July 14, 2016 at 06:06:03
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 I did check some dissimilar triodes tubes. 6DE7 is used in Woo audio headphones amplifier. It must not be bad...

 

RE: Input tube suggestions?, posted on July 14, 2016 at 09:04:29
Donald North
Manufacturer

Posts: 1296
Joined: February 8, 2001
 There are many dissimilar dual triodes with differing parameters for the two sections. I think they can make good sounding amplifiers - the first amp I designed on my own used the 6DN7, a tube of which Paul Joppa has a lot of experience.

 

RE: Input tube suggestions?, posted on July 14, 2016 at 10:39:23
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 PAUL! HEY PAUL!

What do you think of 6DN7 for driving a 300B? :)

 

RE: Input tubes, posted on July 15, 2016 at 00:27:55
aknaydenov
Audiophile

Posts: 106
Location: Sofia, Bulgaria
Joined: September 13, 2015
 If you insist on a 2-staged amplifier, I can recommend D3A and or triode strapped. Especially the later has a mu of 68 and an Rp of 1,5k in triode mode. I'm currently using it on my 6P45S SET and I love its sound.

 

RE: Input tube suggestions?, posted on July 15, 2016 at 09:13:00
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016
 I want to thank everyone for all replies until now.. More than I expected!
Many options are "on the table"...

The easy way is...unbypassed cathode resistor for 6EM7 high µ triode for lower gain (a bit over 20dB).

If 6EM7 high µ triode is not used:

Around µ=20: 6SN7, 6J5, 6C5 (similar to 6J5 with slightly higher plate resistance and lower transconductance), 2C22/7193...

Around µ=30: 6DJ8, 6922, 6N23P...

Those two last categories could be used with CCS and LED biasing.

Also, a different dissimilar triodes tube with different specs is possible, like 6DE7. However, how is it linear for an adequate gain?

Finally, some guys suggested a completely different approach. Even if their results are quite good, it wouldn't be my first choice.

 

RE: Input tube suggestions?, posted on July 15, 2016 at 10:44:05
fred76
Audiophile

Posts: 1586
Location: Manila
Joined: February 28, 2004
 Hi,

That looks good.. You may want to try the 6BM8/ECL82 triode-pentode in a similar scheme using an IT with the pentode section triode strapped.. You may get away with unbypassed cathodes for the high mu triode sections too.. IRRC, Cyrus Brenneman used the same input-driver for one of his SE amps but choke loaded instead of IT's.. Good luck.

 

RE: Input tube suggestions?, posted on July 18, 2016 at 18:37:21
Genp
Audiophile

Posts: 57
Location: Not far from Montreal
Joined: February 19, 2016



Hi.
Maybe the 6922/6EM7(L-µ section) combo could be interesting.
(Keep in mind 6EM7 H-µ section with an unbypassed cathode resistor)
This is how it looks:

6EM7 Low-µ
operating point: 160V, 42mA, -24V
plate diss.= 160V x 0,042= 6.7W
rp= 900, S= 5.6 mA/V, µ= 5
Rk= 24V/42mA= 571....560R

6922/E88CC (according Telefunken datasheet)
operating point: 90V, 6mA, -2.2V
rp around 4k, S= 8mA/V, µ= 30.5
plate dissipation=(90V x 6mA)= 0.54W
drawing line from 250V leads to 9.3mA...
...giving plate resistor of 27k
Rk= 2.2V/6mA = 367...360R
Gain= (µ x Ra)/(Ra+rp), Ra=22.9k (27k II 150k)
= 26
The 39µF cap should be Solen, Axon or similar




 

Webster output transformers, posted on November 20, 2019 at 08:18:52
wclarson12
Audiophile

Posts: 30
Location: Hudson Valley NY
Joined: April 10, 2014
 Hello all. I just purchased a Webster organ amp on Ebay.
The Webster outputs are gapped.
Has this already been determined?

 

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