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Old 14th September 2010, 12:02 PM   #21
45 is offline 45  Italy
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Also, if you can get 275V supply you could go for back-bias just using a 220 ohm resistor between the first and the second PSU capacitors. The cathode of the second cap is grounded while the cathode of first cap and the bridge are leaft floating. With the full current of the amp flowing through that 220 ohm resistor (say, 100 mA) you will get apprx. your 250V anode voltage and enough bias to play with.

Another solution could be 7x1.5V batteries in series.

Cheers,
45

P.S.
All the 6V6 I have, when put on the Hickok 539C as from manual hit the top of the gm scale at 6 mA/V and in some cases I must switch to 15 mA/V scale...!!!

Last edited by 45; 14th September 2010 at 12:07 PM.
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Old 14th September 2010, 04:45 PM   #22
kevinkr is offline kevinkr  United States
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Originally Posted by 45 View Post
Also, if you can get 275V supply you could go for back-bias just using a 220 ohm resistor between the first and the second PSU capacitors. The cathode of the second cap is grounded while the cathode of first cap and the bridge are leaft floating. With the full current of the amp flowing through that 220 ohm resistor (say, 100 mA) you will get apprx. your 250V anode voltage and enough bias to play with.

Another solution could be 7x1.5V batteries in series.

Cheers,
45

P.S.
All the 6V6 I have, when put on the Hickok 539C as from manual hit the top of the gm scale at 6 mA/V and in some cases I must switch to 15 mA/V scale...!!!
I tried back bias and while it did work I felt it was needlessly complex given the intent of this design and I removed it. The current 12V batteries (A23 type) are very inexpensive, extremely compact and will last practically "forever." (3 - 5yrs until replacement) They're pretty ideal IMHO for this application as I really don't want much cathode degeneration. There is just a modicum of cathode bias generated by the 10 ohm cathode resistor which I also use to monitor the operating point. (It is quite stable and around 48mA - 50mA total per tube)

The amplifier is actually running and largely complete - I will replace the 200 ohm resistor in the supply with a choke as I need more ripple suppression. Any other changes will focus mainly on improving the driver stage, feedback implementation and concerns about stability. I may ultimately spring for a better set of OPTs - these are very good, but I suspect I need more primary inductance because of the relatively high rp of the 6V6 even with primary side fbk applied.

Mostly I need to calm down and do some measurements..

My 6V6GTs are U.S. made 1940s vintage Kenrad and NU (National Union) and may differ significantly from modern (foreign) production. They are very close to the stated conditions in the RC-12 tube manual when running at 250V and effectively -12.5V of bias.

I have a Hickok 539B.. Very nice tester..
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Old 14th September 2010, 05:24 PM   #23
Cassiel is offline Cassiel  Libya
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I have never designed or built an SE pentode amp before, and decided I would do all pentode for a change.
OK, understood. I have tried a pentode driver for a 2A3 amp (Yamamoto design) with good results. An all pentode amp may be too much for my taste (triode fanatic here) but you never know until you try. 6J1 sounds good in triode mode, don't know about the 6J7 - in case you want to triode connect that one later.
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Old 14th September 2010, 06:03 PM   #24
45 is offline 45  Italy
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My 6V6GTs are U.S. made 1940s vintage Kenrad and NU (National Union) and may differ significantly from modern (foreign) production.
I have a quad of JAN-RCA 6V6GTY 1958 with grey glass and true micanol base (very nice, indeed!) and a pair of Sylvania VT-107 1940's coke bottle and grey glass.

The Europeans test the same, they are Italian Fivre 6V6GT with black glass from the early 70's. They have a slightly bigger bulb than the American versions but it is just this production. In fact the older 1950's type was very close to the RCA in every detail...

All these test around 5.5 to 6 mA/V and 40-42 mA.

Then I have some Russians which test unusually strong up to 7.5 mA/V at 45 mA!

Cheers,
45
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Old 14th September 2010, 06:15 PM   #25
kevinkr is offline kevinkr  United States
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OK, understood. I have tried a pentode driver for a 2A3 amp (Yamamoto design) with good results. An all pentode amp may be too much for my taste (triode fanatic here) but you never know until you try. 6J1 sounds good in triode mode, don't know about the 6J7 - in case you want to triode connect that one later.
Definitely in the triode fanatic camp myself, which is particularly why I designed this amplifier. Good once in a while to re-evaluate one's stance about such strongly held beliefs.

What I am hearing is much better in some ways than I expected, but there are also significant audible deficits. As always a bunch of trade-offs are involved. It is certainly very dynamic and punchy in a way that belies the low power rating. Detail, imaging and depth would all fit the exceptional category. Tonal balance tends to the hard, almost but not quite solid state sounding end of the spectrum. Bass is insufficient, but what there is sounds right enough. It is a little pushy on some material, and fatiguing to listen to on others. (In fairness that particular material was not much better sounding with an all triode amp.)

Makes me think that combining the colorations and strengths of the two might be a winning combination - combining strengths and offsetting weaknesses. Something I will look at after I complete the next amp which is a IT coupled GM70.
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Old 14th September 2010, 06:19 PM   #26
kevinkr is offline kevinkr  United States
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Originally Posted by 45 View Post
I have a quad of JAN-RCA 6V6GTY 1958 with grey glass and true micanol base (very nice, indeed!) and a pair of Sylvania VT-107 1940's coke bottle and grey glass.

The Europeans test the same, they are Italian Fivre 6V6GT with black glass from the early 70's. They have a slightly bigger bulb than the American versions but it is just this production. In fact the older 1950's type was very close to the RCA in every detail...

All these test around 5.5 to 6 mA/V and 40-42 mA.

Then I have some Russians which test unusually strong up to 7.5 mA/V at 45 mA!

Cheers,
45
How do those Russian ones sound?
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Old 14th September 2010, 06:46 PM   #27
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Definitely in the triode fanatic camp myself, which is particularly why I designed this amplifier. Good once in a while to re-evaluate one's stance about such strongly held beliefs.
It is called now "Fix your knowledge"

By the way, 6J7 and 6SJ7 are very nice long forgotten tubes.

6П6С is a fair 6V6 copy, but has wide spread of parameters like all Russian tubes.
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Old 14th September 2010, 07:53 PM   #28
45 is offline 45  Italy
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How do those Russian ones sound?
They sound fine. As a power tube I have only listened to guitar amps. In HiFi equipment I have once listened to a fully battery operated preamp of a friend of mine! In fact you can run them nicely at very low plate voltage. That worked at around 30V in triode connection (bias about -1.5V for 3.5 mA) 10K load (LC coupled).

Cheers,
45

Last edited by 45; 14th September 2010 at 07:56 PM.
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Old 14th September 2010, 08:24 PM   #29
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An all pentode amp may be too much for my taste (triode fanatic here).......Definitely in the triode fanatic camp myself, which is particularly why I designed this amplifier. Good once in a while to re-evaluate one's stance about such strongly held beliefs.
I have been in the triode ONLY camp for like forever, except for guitar amps (pentodes just scream better). I was in search of a 100 WPC triode only P-P amp, which was found during the development of the amp discussed in the 6L6GC in AB2 thread.

Then I bought one of Pete Millett's red boards. It contains NO TRIODES, none, nada, zip. I can't stop playing with it. It just rocks better than most other amps. Of course I have hot rodded it into stratospheric power levels (250 WPC) but it sounds great as delivered. It is a different sound than a SET for sure, but a welcome change especially for loud or dynamic music. Don't dismiss the pentode until you try it for yourself.
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Old 14th September 2010, 08:35 PM   #30
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My first Pyramid amp used triode input, triode Concetina splitter, double triode LTP driver, pentode output.
Second version used pentode-triode (pentode input, triode Concertina splitter), triode LTP driver, pentode output.
The last version uses almost all pentodes: pentode input, triode Concertina, pentode LTP driver, pentode output.
No difference except optimal amplification factor between all 3 versions. All sound as very good PP amps.
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