• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Vacuum Tube OTL power amp!!

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Sounds like you're enjoying it M-Gregg....

I'm still messing round, Ive made up some 'CCS' (MOSFET Anode-Load boards), but as the volts from them is approx 1/2 +B to allow the best op-point of the SN7 before the phase-splitter, I must use a pot. divider or modify the P.I. for self-bias, to keep the P.I. bias point right.....

I prefer the pot. divider idea,--two caps in the gain-stage is quite enough IMHO! although this will lower the loop-gain by a few dB....

--Hey M-Gregg, We may have a use for those 6SL7 you have after all!
 
Well, Ive done this last modification.

Ive now got MOSFET loaded second gain stage, with a pot. divider to maintain correct bias for the P.I. as its still DC coupled.

First impressions, sound is a tad 'richer' and bass appears to extend further down,--less 'cramped'...

Clarity is superb, and there doesnt appear to be much in the way of lost gain, probably due to the action of the 'Mu stage', the effective amplification-factor is now closer to the theoretical max of 20 for the SN7, whats lost in the pot. divider is sorta partially compensated for.

Much more listening before I decide whether this mod is worth it, but first impressions are good....
 
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Well, Ive done this last modification.

Ive now got MOSFET loaded second gain stage, with a pot. divider to maintain correct bias for the P.I. as its still DC coupled.

First impressions, sound is a tad 'richer' and bass appears to extend further down,--less 'cramped'...

Clarity is superb, and there doesnt appear to be much in the way of lost gain, probably due to the action of the 'Mu stage', the effective amplification-factor is now closer to the theoretical max of 20 for the SN7, whats lost in the pot. divider is sorta partially compensated for.

Much more listening before I decide whether this mod is worth it, but first impressions are good....

Should be interesting!

......:up:

Regards
M. Gregg
 
Well yes you are correct,

I have the replacement tube for the original 6H8C.

The sound is way better. So their you go. Tung Sol's on order. :)

Regards
M. Gregg
You want to watch it with **any** of the Russian-made 6SN7s. So far the Tung Sols have shown a lot of leakage, although they sound OK if leakage is not a problem in your amp (meaning that you have regulated DC supplies for the 6SN7 filaments...).

The vintage Russian 6SN7s are another matter- by 'vintage' meaning 1970s or earlier. They can be excellent.
 
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You want to watch it with **any** of the Russian-made 6SN7s. So far the Tung Sols have shown a lot of leakage, although they sound OK if leakage is not a problem in your amp (meaning that you have regulated DC supplies for the 6SN7 filaments...).

The vintage Russian 6SN7s are another matter- by 'vintage' meaning 1970s or earlier. They can be excellent.

Thank's for the information,

I have regulated supplies for heaters on the input 6SN7's.
I have a vintage (NOS) Brimar in the phase splitter (AC Heater).
The ones I am using on the input at the moment are 1960's russian.

What do you regard as the "Best" 6SN7?

Regards
M. Gregg
 
:sigh:
Thank's for the information,

I have regulated supplies for heaters on the input 6SN7's.
I have a vintage (NOS) Brimar in the phase splitter (AC Heater).
The ones I am using on the input at the moment are 1960's russian.

What do you regard as the "Best" 6SN7?

Regards
M. Gregg

The 1960s Russian tubes are very nice. Most of the American 6SN7s are good too. Certain ones are sought after- the red base RCA, the metal base Sylvania 'Chrome dome' and the Ken Rad...

The Treasure tube sold by Grant fidelity in Canada is supposed to combine the best of the best features of the vintage tubes. I have enough reports from customers that basically say the same thing that I believe them now. However they are not cheap :/
 
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:sigh:

The 1960s Russian tubes are very nice. Most of the American 6SN7s are good too. Certain ones are sought after- the red base RCA, the metal base Sylvania 'Chrome dome' and the Ken Rad...

The Treasure tube sold by Grant fidelity in Canada is supposed to combine the best of the best features of the vintage tubes. I have enough reports from customers that basically say the same thing that I believe them now. However they are not cheap :/

Thank's I know you have been asked to many times!
Just a few quick questions .LOL

Any thoughts on how long a 6C33C lasts in audio at app 180-200mA?

What is the normal fail condition ie short / low emission / heater fail, I guess this shows as increase in clipping level until unusable?

Has anyone used a new set until they fail?

Just for interest, my experience with burn in of 6c33c is: At the start the bass is quite weak and sound is rough. Then it gets smoother and deeper as the hours pass until it seems to settle with a warm depth of sound that just sounds so natural. I can't think of a Valve that sounds quite like this....strange!

Regards
M. Gregg
 
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At the start the bass is quite weak and sound is rough. Then it gets smoother and deeper as the hours pass until it seems to settle with a warm depth of sound that just sounds so natural.
Probably this is somehow related to Auto Bias. I used 6C33C in OTL amp with Fix Bias only and never noticed such behaviour. No problems with bass at all, it's just excellent from beginning with new tubes.
 
The 6C33C is rated for 1,000 hours at its ratings.

At lower ratings like we are using here, then life is considerably extended. It was designed for rough military service, so is really being 'pampered' when run as an audio-tube, with no rough shocks or vibration.

Ive got 15 tubes. These range from a few tens of hous to many hundreds of hours. None (yet) show any signs of low-emission or other issues.
The getters are still perfect, and there's no apparent other visual differences to when they were new.

They were used in a 'SE' project, in Circlotron, and in this project, over the last few years.

Only failure I had was my own fault relating to not burning the tube in and it arced over. That tube however, still worked afterwards, but I smashed it as you can never trust it again.

As to how long they'll last--How long is a piece of string?--You never can tell, Suffice to say, they'll last a long time!...

I cant say I ever had bass issues, but then again, all my tubes had been used for some time before this project was started...
--It may relate to the way the cathodes age in the first few 10's of hours, developing full emission over this time...
 
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Probably this is somehow related to Auto Bias. I used 6C33C in OTL amp with Fix Bias only and never noticed such behaviour. No problems with bass at all, it's just excellent from beginning with new tubes.

Thank's,

Regards the bass I when I said weak, I mean compaired to the tube after about 10 Hrs. :)

The bass is still very good at the start, however it is much warmer and deeper after about 10 Hrs. I know this because after the inital warm up (burn in) I ran one side for a while, when I switched over to the other half of the tube the difference was obvious. Again after about 10Hrs the warm deep bass came in..Now both sound the same.

Regards
M. Gregg
 
The SE was using some large home-wound transformers....

It sounded OK, but not as revealing, or completely clear as either this, or the circlotron OTL.
--The Circlotron was based on the 'Allan Kimmel Sweep-Tube Monoblock' and was a complicated beast, with more valves than Mullards!.....

Maybe it was the Tx in the SE that I wound, or just the fact it had a Transformer hanging on the O/P, marring the sound, not sure.
I've not tried a 'Proper' transformer by a good maker with this valve, cost being the major reason--I got bored by the SE and dismantled it after some months of messing round with it....
 
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The SE was using some large home-wound transformers....

It sounded OK, but not as revealing, or completely clear as either this, or the circlotron OTL.
--The Circlotron was based on the 'Allan Kimmel Sweep-Tube Monoblock' and was a complicated beast, with more valves than Mullards!.....

Maybe it was the Tx in the SE that I wound, or just the fact it had a Transformer hanging on the O/P, marring the sound, not sure.
I've not tried a 'Proper' transformer by a good maker with this valve, cost being the major reason--I got bored by the SE and dismantled it after some months of messing round with it....

Well it's been running 4Hrs today!
Still sounds Great! :)

Put some spare tubes on the Xmas list....Maybe I will get lucky, and get the tubes as well..LOL...Then again maybe just the tubes...Knowing my luck I will get one to start the set.LOL

Regards
M. Gregg
 
Well, Ive decided that the MOSFET 'Mu-Stage' addition to the second gain-stage is well worth the effort....

Power handling at higher levels is improved, as the op-point of the SN7 is more ideal, this stage was going into clipping before the O/P stage, as it was previously...

Here is the second stage as I have it now--.
 

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