• 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.

Want to build a PP spud.

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20to20

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Joined 2010
Shoog said:
Its starting to look like a 6BN11 PP with 30% Schade feedback around the input transformer, and Garter bias to sort out the current mismatch.
Or possibly the QQv02-6 in the same arrangement.

Shoog
Click to expand...

If you are using an input tranny to replace a VA, you don't need a compactron or a traditional PI section. There is a self-inverting PP circuit that will run Class A. One dual pentode will do it. Are you going for stereo?
 
Shoog

Shoog

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Joined 2002
Really the only thing that would push me towards the QQV02-6 over the 6BN11 is that Compactrons are not available over here in Europe. That means ordering bases and valves from the states - and postage has recently made that an unattractive prospect.
Shef why do you say only the 6BN11 offers the option of Garter bias, surely it would work equally as well on the QQV02-6 ?

I have built a nice little PP amp using a QQV03-10 and a 12L8GT as the driver. It behaves more like a SE design as one side of the QQV03-10 is running as an anti-triode and the other as a Schaded pseudo-triode. I cannot for the life of me hear any sonic penalty for the Tetrode when listening at normal levels.

PS - Love the little SHAX project, are you still impressed with it ?

Shoog
 
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Shoog

Shoog

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Joined 2002
It seems to me that the 6BN11 can be run much harder than its average recommended parameters which only put 11mA through at 125V.
Seems to me that it would be better for this application to run it at between 20-30mA which would require a plate and screen voltage of something like 150V. Has anyone got any experience of running this valve in this sort of range.

Shoog
 
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Shef

Shef

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Joined 2015
Shoog said:
Shef why do you say only the 6BN11 offers the option of Garter bias, surely it would work equally as well on the QQV02-6?
. . .the little SHAX project, are you still impressed with it ?
Click to expand...
Because any tube other than these 2 CTrons mentioned in the topic has the common cathodes. . . quite an inconvenience for Garnet bias, isn't it? ))

I do enjoy that amp each day I'm in the office. I can't say it it truly hi-fi, and the speakers are crappy Minimus7, but overally and at the volumes of 0.2WPC the whole thing sounds warm and pleasant. I listen to mostly modern Jazz and to Funky-ish instrumental which that little box does can deal with.

As of your project, someone already advised where to get CTrons from, get all 4 to have the spares. The ceramic sockets of very decent quality can be purchased for like $10:
4PCS 12 PIN GOLD Ceramic PCB VACCUM TUBE SOCKET FOR 50CA10 AUDIO tube amp parts | eBay
The seller is even close to you than to NA ))

For schematics on QQV02- the only option seems be the fixed bias. For CTrons the Garnet + a common CCS will give the convenience of not worrying of halves' mismatch.
For the input trafos I d think of Hammond 106,107,108 series, check which one would work as 1+1:4+4. The OPTs will be your biggest challenge. You will really need it like 40K p-p to get any decent sound.
Do not waste money on oversized ones, if you expect 1WPC the 5VA-sized are more than enough. I rewinded the OPT in my project, but if you wanna it hustle-free the Hammond 125A or B would work good enough, tested these. They have wide-range variable secondaries, which will help you to find the optimal load impedance. Though, these are lacking the UL taps.
I tried a couple brands of small mains toroids as OPTs but did not like them at all.

Good luck.
 
Shoog

Shoog

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Joined 2002
Shef said:
Because any tube other than these 2 CTrons mentioned in the topic has the common cathodes. . . quite an inconvenience for Garnet bias, isn't it? ))<snip>
Click to expand...

Thanks for all that, I forgot about the common cathode arrangement on the QQ's.

I have the input transf's already, some decent little ones from OEP. The OT wont be as demanding as you might imagine since I will likely be trading off a lot of the gain for lower rp (Schading round the IT), and since leadbelly reports gains of over 150 from this pentode, there should be enough to spare.

Shoog
 
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