DIY Pete Millett Power Amp. I have questions if you have time.

[zebulon1]

I traded a restored PL4000 Autocorrelator for this DIY (P. Millett styled, I think) amp.
It was built several years back and I believe I found the Link that Pete M. has posted concerning this amp.
http://www.pmillett.com/6B4G_push-pull_amp.htm

Can you guys confirm this?
And, what the heck output tubes are the ones installed in this amp? Their like 6B4G's but with the Plate connection on top(?). Edit: their 6A8G's

The amp operates but requires some gain to get up to listing levels and seems to clip easily (Maybe because its missing the second 6SN7 on each channel?). If listening at lower levels' it sounds really nice. I haven't checked it out completely. The friend threw some tubes in it from his crazy mixed up stash, so the tubes condition is questionable.

I plan to rebuild this in a different chassis and cleanup the rats nest of point to point build on the bottom side.

Fairly new to the tube gear and this is the first time re/building something like this.
Please - no comments on whether or not the trade was in my favor

I completely understand the safety concerns for the tube gear and consider myself a pretty good solid state repair tech.

 

[kward]

That's a sweet little class A push pull amp. About 8 watts output per channel.

He made two versions. Pictures of yours looks like the low gain version with just the 6SL7 phase inverter/driver.

Confirm that the small tubes in front are 6SL7s?

 

[Lavane]

Personally I think you did good! I see Autocorrelators for sale all the time if you ever wanted one again. I've heard the original and the changes it made were very subtle. I guess that was the idea, but I wasn't really impressed that much with the Autocorrelator.

 

[zebulon1]

Their 6SN7's

Lavane: On the PL Pre.
The Phase Linear 4000 is a very nice noise reduction pre and the adverb "Subtle", is stretching the ability of the tone controls for sure.

 

[zebulon1]

After looking at the inverters, they might not be the same tube. One is an RCA and the other is a GE. The one is labeled as a 6SN7 but the other is missing the badge.

 

[kward]

Ah my mistake. Both hi and lo gain versions used 6SN7 phase inverter/driver. But it looks like the builder of yours left off the voltage gain stage all together! Interesting. Would be good to validate the phase inverter/driver is wired as a cathode coupled differential pair as the schematic says. But there may be nothing wrong with it, as it puts out low wattage, it may just need high sensitivity speakers.

 

[strabo]

You could just ping Pete directly, he's a hell of a nice guy and if it's his design he'll probably answer any questions.

 

[knockbill]

Plate cap tubes look like 6BG6s, if they are octal bases... if not maybe 807/1625... They don't look like 6A8G
https://www.radiomuseum.org/tubes/tube_6a8g.html

 

[Palustris]

Plate cap tubes look like 6BG6s, if they are octal bases... if not maybe 807/1625

I agree. The 6A8G is a pentagrid converter which is apparently used to make an oscillator. I don't think it is very useful for audio.

 

[zebulon1]

This is one of the output tubes.

 

[gadget73]

looks like a 6BG6G to me.

 

[zebulon1]

looks like a 6BG6G to me.

After looking at a hundred images I confirmed your tube wisdom and they are 6BG6G's. One actually has the designation printing on the side. Arff.
Now I'm trying to find the scat Dave used for the build.