Do you think the OT tap you use makes a difference in tone?

[Mick Bailey]

It's a good while since I've seen one, but there used to be speakers with multiple taps to give an impedance choice. The last I had was dual impedance (4 and 8 ohm) but I've also seen them with three options. I suppose the design is rather inefficient compared to a regular speaker. I wonder what the curves would look like.

[daz]

Here's an unrelated question, but rather then start another thread i'll just ask it here. It just struck me the PT in my amp is a heyboer 18 watt. As i have said previously this amp has gone thru many changes and i guess when it was a EL34 amp with hammond transformers i had bigger filters and when i went to 6V6 i guess thats when i put the 18 watt PT back in. (along with a new OT for the 6V6) I changed the preamp nodes to 22uf but i left the 50/50uf can from when it was EL34. I didn't think about it when i ordered all new electro's to recap it but i ordered another 50/50. Is there an issue using that with the 18 watt PT?

[Helmholtz]

The VA rating of a tube amp PT (PP class AB) should be roughly 3 times the ouput power.
Otherwise the PT is likely to overheat at full output.

[daz]

I was going to post this but forgot..
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[daz]

Would that be 34.8?

[Helmholtz]

Would that be 34.8?

If that means 34.8W, no.
You can't calculate power from DC current and AC voltage.

What is your B+?
What clean output power do you measure?

[daz]

If that means 34.8W, no.
You can't calculate power from DC current and AC voltage.

What is your B+?
What clean output power do you measure?

I just took the PT's 120ma and the 290v and used a online calculator and thats what i got. Output? No idea, as don't u need a scope to measure that? 6V6 plates and B+ at the rectifier output 400v.

[Helmholtz]

I'd say that PT is good for around 20W output.

[daz]

So playing it near full would be a death sentence? The OT is 30w. Guess it's time to look for a new PT. Damn.

[daz]

Wow, prices on transformers are about 3 times what they were last time i bought one ! Anyone know of a PT that would do the job thats nowhere near $150 ?

[Helmholtz]

The 30W OT rating is only an upper limit.
Actual output of your amp should be 20W to 22W max.

[daz]

So exactly what are we talking about as the danger factor? The cap size? The PT is just too small and needs a higher current rating than 120ma? I don't wanna spend a lot of $ if i don't need to because i don't even know if i will be making this my main amp until i try it in a band context, and i can't risk that if it's gonna go up in flames.

[Helmholtz]

Cap size doesn't matter.
The PT might be ok.
To be sure I would need to know the DC current at full output.

[daz]

I can't provide that, as i don't even have a load. I had a 100w load but i have no idea what happened to it. I'll see if i can find it but i haven't seen it since i moved a year ago.

[Chuck H]

This is a really coarse test, but...

Turn the amp on, put it in play mode and let it idle for twenty minutes. Put your finger on the PT. Hot? Too hot to hold your finger on it? No? Good. That means your PT is keeping up with the current load at idle and the filament requirements without much trouble. Now...

Crank it up and play it hard for twenty minutes. Now put your finger on the PT. Hot? Too hot to hold your finger on it? If it is then you may be stressing the PT when conducting max signal under clipping conditions. If you can hold your finger on it it's almost certainly fine.

Heyboer makes pretty sturdy iron. I've used a few of their units and they've held up very well with hard use.

P.S. A better way to know if your filament tap is being stressed is to just measure your filament voltage. If it's more than 10% low you might be stressing it. This shouldn't be the case with your amp.