A Mar 19 Update: Amp techie peeps - how easy is it for you to understand my Sheldon TT3?

TheBigDipper

I've owned a Sheldon TT3 for a couple of years. It must be 10 years old now. A 1W-3W, 1x12 combo. I really like it, but it seems to need some TLC.

• The preamp stage now seems to overdrive almost immediately. I don't remember that from when I first bought it, but in truth, I've not been using it for a clean sound ever since it arrived, until now. I've seen YouTube clips of one being played and getting a clean sound, though. 
• I'm getting a lot of hiss/hum noise when I use the rotary control (it's labelled "Freeze") that changes it from a 1W class A operation (where it is dead quiet - lovely) to 3W class A/B operation. This is audible over the speaker without plugging a guitar in. 

It's a hand-wired amp with the components on a piece of circuit board. How easy would it be for any competent tech to understand it and diagnose the source of the problem? It's slightly unusual in that there are 2 x 12AX7 valves in the preamp stage and 3 x 12AU7 valves in the power amp stage.

https://www.dropbox.com/s/99hy9geyj3ylqkc/IMG_0042.jpg?raw=1

I don't have a schematic, so I don't know how anyone could check if it's using the components it had when it was built. The original company is long-gone and even if the original people could be contacted, I think they deserve the right to leave their past behind if that's what they want to do. They've no obligation to me in the slightest. 

So, could any half-competent tech understand it? Anyone got any suggestions where I might take it? I'm in the Bournemouth area.

Thanks. 

ecc83

That IS an odd configuration! Pretty nicely laid out though so no deent tech' should have a problem sorting it out.

The crunch is cost. The tech' is going to spend quite some time just woring out the circuit and that time needs paying for.  Parts look pretty basic Cs, Rs and valves  and I have an inkling the trouble might be a broken earth return somewhere?

But, A/B operation? Can't see any bias circuits that could do that?

Dave.

RiftAmps

ecc83 said:

But, A/B operation? Can't see any bias circuits that could do that?

Dave.

I'm guessing it brings in another 12AU7 in parallel with the first, seems to be a popular method in the low-watt design community.

Whether it actually reduces the conduction angle to <360 I don't know. Probably not.

ICBM

I would guess it's the usual (and incorrect) idea that single-ended = Class A and push-pull = Class AB, hence the rotary control probably just brings in the other half of the push-pull. This is how other amps with rotary 'Class' controls like the Peavey Valve Kings do (and describe) it.

In which case, the noise is most likely coming from either the power valve which is in use in 'Class AB' and not in 'Class A', or the phase inverter. Whether that's feasible given that there are an odd number of power valves - although an even number of possible power valves if they're not internally paralleled! - I don't know.

But as Dave said, any decent tech could reverse-engineer it in short order, it's not a complex amp.

agibbo

I have the same amp and in the higher wattage mode there is an audible hum from mine too. As far as I can remember it was like it when I got it, probably around 7 years ago or so!

As far as clean sounds, there is very little clean headroom with humbuckers, but singlecoils are fine.

TheBigDipper

agibbo said:

I have the same amp and in the higher wattage mode there is an audible hum from mine too. As far as I can remember it was like it when I got it, probably around 7 years ago or so!

As far as clean sounds, there is very little clean headroom with humbuckers, but singlecoils are fine.

Thanks for the feedback on the hum. I actually don't use the 3W setting normally, but the hum was just something I noticed and started to wonder about. The power amp valves don't have shielding covers and are quite close to the speaker coil. I wonder if that is something to think about.

Regarding the headroom - even with 1973 Strat pickups (the lowest o/p I own) I'm hardly able to turn the Gain control very far before I can hear the tone starting to crunch up. I'm not looking for Fender Twin cleans, but this isn't how I remember it from when I first bought it - but I've been choosing crunchy tones ever since and only started looking for cleans a little while ago. 

agibbo

The way I set mine when trying to get a decent clean tone with humbuckers is volume all the way up, the freeze tone set just before the hum becomes audible (about two thirds the way up) and then bring the gain in very slowly, keeping it clean, and you should get a nice chiming clean tone. For a bit less volume (if it's too loud) just turn the freeze control down! 

ecc83

agibbo said:

I have the same amp and in the higher wattage mode there is an audible hum from mine too. As far as I can remember it was like it when I got it, probably around 7 years ago or so!

As far as clean sounds, there is very little clean headroom with humbuckers, but singlecoils are fine.

Ah, one of the problems with a triode output stage is that they are more prone to HT line ripple than pentodes (so long as the G2s are well smoothed) . You might get lower hum with a better matched set of 82s but if the caps are too small they will need beefing up.

Dave.

ecc83

agibbo said:

I have the same amp and in the higher wattage mode there is an audible hum from mine too. As far as I can remember it was like it when I got it, probably around 7 years ago or so!

As far as clean sounds, there is very little clean headroom with humbuckers, but singlecoils are fine.

Ah, one of the problems with a triode output stage is that they are more prone to HT line ripple than pentodes (so long as the G2s are well smoothed) . You might get lower hum with a better matched set of 82s but if the caps are too small they will need beefing up.

Dave.

ecc83

Sorry! I am still having trouble posting. Have gone back to IE but posts still "hang".

Dave.

But fine THIS time!

jpfamps

ICBM said:

I would guess it's the usual (and incorrect) idea that single-ended = Class A and push-pull = Class AB, hence the rotary control probably just brings in the other half of the push-pull. This is how other amps with rotary 'Class' controls like the Peavey Valve Kings do (and describe) it.

In which case, the noise is most likely coming from either the power valve which is in use in 'Class AB' and not in 'Class A', or the phase inverter. Whether that's feasible given that there are an odd number of power valves - although an even number of possible power valves if they're not internally paralleled! - I don't know.

But as Dave said, any decent tech could reverse-engineer it in short order, it's not a complex amp.

The Sheldon amps got all their ideas from Kevin O'Connor's Ultimate Tone books (the tech info on their website was lifted verbatim including the mistakes!!), so the A to AB control with almost certainly be identical to the patented Peavey control, as Kevin has pointed out that the patent is identical to the schematic in his books!

TheBigDipper

An update.

I stumbled across a retired chap near me (he's in Hythe, Southampton - trading under the name Retro Workshop). We had a chat about the TT3 and I went round to show it to him. And then left it with him. A week later, I've picked it up and it no longer hums like it used to. Not totally quiet at full volume, but such an improvement over the original noise. As far as I can tell, tonally just as nice as it was before. Still breaks up earlier than I remember - I didn't ask him to do anything there - but I've found a nice clean sound. 

So, what did he do? I paid for him to trace the circuit and report back. I suspect he spent 2-3 times longer on it than I paid for - he was quite intrigued by the project. In reporting back, he said he though it was a "catalogue amp" schematic and had a design flaw regarding the HT side of things. That ties back with what @jpfamps and @ecc83 ;posted. He said he'd worked out a quick and inexpensive solution to that (£20-£25 parts & labour) so I told him to go ahead and do it. Which he did.

He also had some things to say about the blurb from Sheldon and endorsed (if I understood it correctly) what @ICBM said regarding Class A vs Class AB and Single-Ended vs Push Pull. He said that the 3W mode was using both sides of the signal and turning the control back to 1W mode just progressively took one half of the signal out of the circuit. He said he put an oscilloscope on it and could see the top half of the sine wave he was presenting the amp et smaller and smaller until it was flat.  

None of which changes anything about how nice the TT3 is - especially now it's quieter in 3W mode. A fine 'home' and recording amp.

So, @agibbo might be interested in this, too.

Thanks to all for your advice and input. Next is find a new, lower power 12in speaker and swap out the V30 it has fitted, to see if that is more pleasing or not. I'm not looking for anything new, so let's see what pops up! 

agibbo

@TheBigDipper, did your tech document what the changes he made were? I'd be interested in getting mine checked over too.

As for a different speaker, I've got a 1x12 cab with a Greenback in it, and I have to say that I prefer the stock V30. However, both sound good so it's just my preference. 

MikeP

I have one of these and had it repaired at one point by Mark at Sheldon amps. He recommended 3 X 12BH7 in the power side you bring the power up to around ten watts and increase headroom. 

TheBigDipper

agibbo said:

@TheBigDipper, did your tech document what the changes he made were? I'd be interested in getting mine checked over too.

As for a different speaker, I've got a 1x12 cab with a Greenback in it, and I have to say that I prefer the stock V30. However, both sound good so it's just my preference. 

I didn't ask. It would have floated over my head. Happy to email you the circuit diagram I now have - but I don't know if it's before or after the mod. PM me and I'll email it. I'm not minded to chase Peter down any further - I think he's given me much more time than he billed already. In a nutshell, though, he said it was clearly a design flaw on the HT side of the circuit and an easy, obvious fix. It's not "hi fi" quiet, though. There is still a hum when volume is on full and freeze is at 75% or higher - just nothing like it used to be.

hywelg

TheBigDipper said:

Next is find a new, lower power 12in speaker and swap out the V30 it has fitted, to see if that is more pleasing or not. I'm not looking for anything new, so let's see what pops up! 

Some of the Jensen range are very low efficiency. I have 4 x  P10R's in a cab and its noticeably quieter than my other cabs.