Marshall clone, what to do?

[Randall]

"DO NOT CHANGE THE AMP!!! FIX IT!"

OK, but what does this mean? Seriously what exactly is broken here?

[Chuck H]

Nothing is broken. That's part of the problem. And since nothing is broken, changing anything about the amp design in an attempt to "do something" is blind shooting. Here's how I see it, from post #42

Randall did the same thing by taking in a repair based on "It lost something". Taking that repair is an agreement to try and put that "something" back.

I'm trying to help, honestly. And I think if you start changing the design or any components that aren't bad you will complicate your situation and have a harder time of this because you will be straying far from where the amp was when the owner liked it. "FIX IT!" in this regard then means to make it like it was when the customer liked it. Changing the design or any component values can only take you further from that.

After going through the thread I've tried to suggest what I think is the next best course of action. For your consideration and with the intention of making your life better.

[nosaj]

"DO NOT CHANGE THE AMP!!! FIX IT!"

OK, but what does this mean? Seriously what exactly is broken here?

Seems to me Chuck was suggesting adjusting the bias to put the sparkle back in.
Back to car analogies. If the car's not idling right you don't go changing all sorts of things. You'd start with the carb and the air fuel mixture.
A carb with the idle set wrong is not broken just needs adjustment, same here most likely.
Tubes not idling right can sound rather $hitty.

nosaj

[Randall]

But the tubes are idling all at 16 - 18 watts, which is ~70%. Is that bad in any way?

[nosaj]

But the tubes are idling all at 16 - 18mA which is ~70%. Is that bad in any way?

Ok think of it this way what were they idling at when the customer really liked it? You can't really know. But if you have him there and let him play while you adjust the bias a little hotter I bet you may find it.
70% is safe but not always what sounds best to players.

nosaj

[Chuck H]

But the tubes are idling all at 16 - 18mA which is ~70%. Is that bad in any way?

There's nothing wrong with that. And there's nothing right with that. I wish whoever invented the 70% rule would stub their toe. Repeatedly. On consecutive days for a week. "Proper" current @ bias is ONLY dependent on safe operating parameters for the tube and the amplifier components. The datasheets commonly exceed 70% for AB and B operation. Drive voltage seems to be the lynch pin. Sited on the data and also my own experience. If you choose to try this I would suggest checking screen current under overdrive conditions be sure the screens aren't overdissipating. But know that it's completely OK for the screens to be right at max dissipation. Remember that with a signal generator you have a constant current. In an actual music program the screens will get a lot of breaks

[bob p]

70% is no big deal. Tubes get biased at 100% for Class A.

[Enzo]

You may have tubes set up for "70%", but one set of tubes at 70% may not sound like a different set at 70%. That is why the suggestion is to adjust them to the owner's ear rather than yours.

There is nothing wrong with 70%, but there is nothing "correct" about it either. The fact the old ones were set for 70% doesn't mean the new ones want to be there.

[g1]

I'm opposed to the default assumption that if it is biased at 70%, and he's complaining about the tone, then it was probably biased hotter before.
70%, in my experience with older stuff (typical A/B), is hotter than normal stock value. Maybe it was biased at 55% before and he liked that.
Just saying that it could go either way, and there seems to always be a 'hotter is better' inclination that I don't think is backed up by historical setting data.

[g1]

But the tubes are idling all at 16 - 18mA which is ~70%. Is that bad in any way?

In complete opposition to what I just said , 18mA is mighty cold, nowhere near 70% at any kind of realistic plate voltage. What are the correct numbers?

[Chuck H]

You beat me to it. I was thinking about posting to say just that the bias should be tried in both directions. My reason for implying hotter before wasn't a 'hotter is better' thing. I was considering that older tubes do tend toward more current than less and also considering that the amp owner didn't think the amp sounded too bad through a bass cabinet, implying a lack of top end fidelity in the old tubes? Also sometimes a consequence of a hotter bias. So it was strictly an interpretation of circumstances, not a guide toward better tone. Indeed I'm now thinking I may have mistaken the bass cabinet thing since modern bass cabinets can have plenty of top end. So maybe it's a fidelity issue in the OTHER direction and the amp needs to be biased cooler.?. Either way... The new tubes may not be biased like the old ones (or otherwise feel/sound different).

[Randall]

"In complete opposition to what I just said , 18mA is mighty cold, nowhere near 70% at any kind of realistic plate voltage. What are the correct numbers?"

Whoops! I meant to say 16 - 18 watts. Current is 34 -38 mA at 484 Vp. I'll go fix that.

[g1]

Ok. Usually most types of 6L6 variants are considered 30W tubes, so 18W puts you at 60%.
Is that where is was when he brought it to you?

[Randall]

It is how it was brought to me. I always considered EL34s 25 watt tubes. Am I wrong there?

[Chuck H]

It is how it was brought to me. I always considered EL34s 25 watt tubes. Am I wrong there?

Nope. Why? Does that mean they need to be biased at a fixed amount?