Dude, where's my brain?

NickS · Post by **NickS** » Thu Oct 02, 2008 12:05 pm

Jeez. Like the guy on AGA just said, check the tubes first.

I've been looking at this noisy 9-year-old Marshall TSL100 intermittently for my son. Replaced the out-of-spec 220K EHT and screen grid divider resistors and recapped the EHT in passing, replaced the 220K stopper resistors with 5k6 as per later issue units, which made it possible to actually bias the damn' thing. Still noisy, sounding like a kettle coming to
the boil in reverse - starts hissy with occasional squeals and pops like wet wood in a fire, gradually quietening to a dull grumble as the thing warms up. It has 4 JJ E34Ls, one newish (result of a liquid spill through the vent, by the look of it).

Didn't matter how many OP valves (tubes) were in (apparently), whether they were in pairs or not, whatever, as long as there was at least one OP valve in, there was noise so I assumed it was a common noise source. Checked valve
bases and PCB for tracking. Substituting resistors and caps had no effect. Then I found that "output mute" (which connects the two out-of-phase inputs to the OP valves) with the 5k6 stoppers in place now pretty much muted the
noise if you had a pair of valves in opposition. Last night I actually put a dual-trace scope on the stopper resistors of two valves and realised that there was different noise on each. The light finally dawned. I'd never actually put the newest of the four JJ E34Ls in on its own. That's it. 3 out of the 4 JJs are noisy crap, the new one's OK.

Post by **Mike** » Thu Oct 02, 2008 12:57 pm

Is it ever a good idea to just put in one valve into a push-pull power amp?

There's no real way to calculate what impedance you should connect to it in that state.

NickS · Post by **NickS** » Thu Oct 02, 2008 1:12 pm

Mike wrote:Is it ever a good idea to just put in one valve into a push-pull power amp?

There's no real way to calculate what impedance you should connect to it in that state.

Impedance isn't a problem; the turns ratio doesn't change. If you think about it, assuming Class AB operation, when one side is in cut-off the other side is driving the speaker on its own. Anyway, I'm just listening to a few milliwatts of noise.

You can even whack the bias current up and get limited single-ended operation (probably with shortened tube life). Or do it properly like Matamp.

Post by **Mike** » Thu Oct 02, 2008 1:14 pm

If you drop from four tubes to two, you absolutely need to change the transformer tap you use. The same would be true of one tube as opposed to 4.

As for using one side of a PushPull, that's a dark world to me, it would sound Bad.

benecol · Post by **benecol** » Thu Oct 02, 2008 1:14 pm

My eyes are seeing words but my mind cannot comprehend what they mean. NickS for President.

NickS · Post by **NickS** » Thu Oct 02, 2008 1:45 pm

Mike wrote:If you drop from four tubes to two, you absolutely need to change the transformer tap you use.
The same would be true of one tube as opposed to 4.

You'd use the same as if you were using two tubes; you have the same number of valves on each half of the winding (i.e. 1) and the same ratio of turns multiplying the load impedance into the anode (plate) load. However, Fender say a 2:1 mismatch is pretty much OK so I just leave the 8 ohm cab plugged into the 8 ohm socket while I'm listening to a few milliwatts of noise.

benecol wrote:NickS for President.

No, nothing would ever get finished. Mike has drive and determination and Gets Stuff Done ^tm. Mike for President.

Post by **Mike** » Thu Oct 02, 2008 1:48 pm

NickS wrote:
Mike wrote:If you drop from four tubes to two, you absolutely need to change the transformer tap you use.
The same would be true of one tube as opposed to 4.
You'd use the same as if you were using two tubes; you have the same number of valves on each half of the winding (i.e. 1) and the same ratio of turns multiplying the load impedance into the anode (plate) load. However, Fender say a 2:1 mismatch is pretty much OK so I just leave the 8 ohm cab plugged into the 8 ohm socket while I'm listening to a few milliwatts of noise.

It's quite possibly fine, but Fender overengineer their transformers (or used to) like whoa. So you could take something that has a silly 4 ohm old tap (Bassman etc) and run it with an 8ohm cab fine. The modern Marshall stuff (TSL series in particular) I wouldn't be so keen to mismatch on at all, although as you say, you're not diming anything.

As regards President, you're too kind, I'm more just "has lots of spare time, or used to". I think this project you're undertaking here is far more complicated than everything I've done put together. Valve amps still terrify me a great deal in terms of the magnitude of the building task.

Servicing a multi-layer PCB beast Tube amp? That has me cowering under the covers.

NickS · Post by **NickS** » Thu Oct 02, 2008 4:53 pm

Mike wrote:Servicing a multi-layer PCB beast Tube amp? That has me cowering under the covers.

The TSL 100's only two-layer with plated-through holes but difficult to get at anything, bring back turret boards!

The multi-layer stuff I mentioned is in computers, where taking a blown electrolytic out of a motherboard can be a right pain. Which reminds me, I need to order some more, my desktop is taking about 5 minutes to get going.