UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

Diabolical Artificer · 29th Dec 2018, 7:59 am

There's some good stuff on there Andy, I've seen it before but thanks anyway.

Just when you thought you'd finished..... I did a bit of testing on one amp as it had quite a bit of hum and was stunned to find what I thought were two 120w mono block's were in fact 40w monoblocks. I really doubted my sanity there for a minute, I KNOW I saw 33v RMS on my bench meter lot's of times but as this has been playing up on AC volts the last few times I used it, it is now suspect.

This and a few comment's from more experienced builders like Tricomp above has made me look again at the circuit. Firstly the front end with local FB cause instability at SW off with a speaker load, this can be sorted by the zobel on the OP, but it would be better if the instability wasn't there.

Next the direct coupled LTP to cathode followers has been giving gyp from day one. Because it is direct coupled the CCS just doesn't act like it should. It should be possible to adjust it with a ECC82 to bias the LTP to around -10v Vgk, it isn't. Somehow the following stage is affecting the LTP I think, though I haven't sussed out how yet.

So that aside, I'm going to re-design from scratch, lost the cathode followers, the big problem being that there are three B9A valve sockets on the chassis, so if I use one or two triodes as the front end, into a LTP, i have one socket doing nowt, seems daft to build a design just to fill that base. Might be room for a EF91 CCS Richard?

Another issue is the PSU de-coupling, which needs improvement. If there is any slight ripple on the front end HT, it's amplified by a high gain stage and goes on to be further amplified. So lot's to think about.

I've had a break from working on them over the hols as it was doing my head in.

Whilst we're here I'd like some opinion's on this blokes MO of using twin triodes in parallel or triode strapped EL84's to drive his EL34 amps, see attached.... This may be a good way to drive the OP stage though I'm a bit wary of strapping two unmatched triodes together; problem?

Andy.

29th Dec 2018, 7:59 am	#48
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: 120W PP EL34 amplifier re-born. There's some good stuff on there Andy, I've seen it before but thanks anyway. Just when you thought you'd finished..... I did a bit of testing on one amp as it had quite a bit of hum and was stunned to find what I thought were two 120w mono block's were in fact 40w monoblocks. I really doubted my sanity there for a minute, I KNOW I saw 33v RMS on my bench meter lot's of times but as this has been playing up on AC volts the last few times I used it, it is now suspect. This and a few comment's from more experienced builders like Tricomp above has made me look again at the circuit. Firstly the front end with local FB cause instability at SW off with a speaker load, this can be sorted by the zobel on the OP, but it would be better if the instability wasn't there. Next the direct coupled LTP to cathode followers has been giving gyp from day one. Because it is direct coupled the CCS just doesn't act like it should. It should be possible to adjust it with a ECC82 to bias the LTP to around -10v Vgk, it isn't. Somehow the following stage is affecting the LTP I think, though I haven't sussed out how yet. So that aside, I'm going to re-design from scratch, lost the cathode followers, the big problem being that there are three B9A valve sockets on the chassis, so if I use one or two triodes as the front end, into a LTP, i have one socket doing nowt, seems daft to build a design just to fill that base. Might be room for a EF91 CCS Richard? Another issue is the PSU de-coupling, which needs improvement. If there is any slight ripple on the front end HT, it's amplified by a high gain stage and goes on to be further amplified. So lot's to think about. I've had a break from working on them over the hols as it was doing my head in. Whilst we're here I'd like some opinion's on this blokes MO of using twin triodes in parallel or triode strapped EL84's to drive his EL34 amps, see attached.... This may be a good way to drive the OP stage though I'm a bit wary of strapping two unmatched triodes together; problem? Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.