Hiwatt power tubes question

[Chuck H]

My preferred tube info search page https://bms.isjtr.ro/search.html
Don’t even need to hit ‘return’!

I like it

[Chuck H]

The KT66 is described as a "beam pentode" with a max Vp of 500 and a max diss of 25W.

The kt77 is described as a "beam tetrode" with a max Vp of 800 and a max diss of 28W.

You have 503Vp at what is already significant current that should be reduced a little (by doing the voltage doubler bias mod). As g1 indicates, this is a common solution to an old amp operating in new conditions. It's a common addendum to many HW circuits because they don't work right otherwise.

Knowing how the problematic it's been in recent history to load current production big bottles into higher voltage vintage amps, this would be all the information I needed to avoid putting kt66's into that amp. JM2C and YMMV.

I would probably go with Ruby EL34-B tubes. These are just Ruby selected Shuguang tubes. They're the only ones that held up and sounded good in a recent build of mine with 480Vp.

[Pedro Vecino]

You have 503Vp at what is already significant current that should be reduced a little (by doing the voltage doubler bias mod). As g1 indicates, this is a common solution to an old amp operating in new conditions. It's a common addendum to many HW circuits because they don't work right otherwise.

I d´ont know if I understood correctly but the DR504 does not need a voltage doubler. It´s enough to extend the voltage divider to the good side with an adjustable resistor. It's the DR103 that needs it.
My advice is to use low-grade EL34 to get some compression. Otherwise they are hard like a stone.
The low grade EL34 (B-STR - Modern Shuguang) I buy them through TAD selecting lower PC number (relative plate current). I imagine that with Ruby it can also be done.

[Chuck H]

I d´ont know if I understood correctly but the DR504 does not need a voltage doubler. It´s enough to extend the voltage divider to the good side with an adjustable resistor. It's the DR103 that needs it.

You're absolutely right. I didn't look at the schematic and took g1's post at face value. Looking at the schematic I see that the bias supply for the dr504 has room for adjustment in the voltage division. Thank you for the clarification.

[Randall]

"I d´ont know if I understood correctly but the DR504 does not need a voltage doubler. It´s enough to extend the voltage divider to the good side with an adjustable resistor."

Could this be explained further? I am not sure I understand your meaning here.

[Pedro Vecino]

Yes. It appears in the schematics of M. Huss.



The voltage doubler is only essential in the DR103 (100 watts). Although it also serves the system that Marshall uses in the JCM900. The latter is simpler, with fewer components.
I d´ont have pics of the bias circuit in the DR504 because it´s very simple. In the DR103 the voltage doubler can be quite laborious if you want to do with the original style of the amp. I keep this image as a reference:

[Randall]

Why is the diode changed in the bias mod? Would a 1N4007 do the job?

[Pedro Vecino]

It´s not necessary. A 1N4007 works perfectly.

[Randall]

Customer opted for JJ KT77s, and rated at 800v, works for me. JJ datasheets sometimes get lost on me. Looking at this one, is this a max diss. 25w or 28w?

https://www.jj-electronic.com/images...s/pdf/kt77.pdf

I did that bias mod, and man was that a chore. I did it topside so as to not disturb the wiring.

[Justin Thomas]

The plate dissipation is 25W. What the sheet means by that is that the total of plate AND screen dissipation shouldn't exceed 28W total. So any combination where Plate <25W, Screen <6W, AND Plate+Screen <28W is good.

Justin

[Randall]

So, when measuring with a socket type bias probe or across a 1 ohm cathode resistor, I am measuring total dissipation, right? Plate and screen?

[g1]

Yes, both plate and screen current run through the cathode.
If using socket type bias probe, you need to verify it is measuring cathode current, some do measure plate current only.

[Randall]

yes, this one breaks the connection at pin 8 and routes it thru your ammeter.

[pdf64]

Customer opted for JJ KT77s, and rated at 800v, works for me. JJ datasheets sometimes get lost on me. Looking at this one, is this a max diss. 25w or 28w?

https://www.jj-electronic.com/images...s/pdf/kt77.pdf...

I think that the Wa,g2 limiting value is intended for UL applications; for this Hiwatt it's the Wa 'plate only' limit of 25W that's relevant.