diode and cap quest

[J M Fahey]

Interesting.

Maybe then that servo bias circuit gates power tubes off in case of detected lack of load.

Didn't analyze the circuit, just thinking aloud.

[trem]

On a "side note" of interest.....this amp runs 2x EL34.
The back panel says it is a 20 Watt amp.!
Have not been able to measure any voltages yet, so I have no idea, but I am wondering where that number comes from.?
Would Blackstar be running this amp at very low plate voltage for some reason.?
Well.......the reason would be to get power reduction. Guess what I am really wondering is.....what is their strategy with this amp...why the 2x El34 power section for a 20 Watt amp.?
Thank You

[Jazz P Bass]

Interesting.

Maybe then that servo bias circuit gates power tubes off in case of detected lack of load.

Didn't analyze the circuit, just thinking aloud.

I have seen that in a H&K mini valve amp.

In that case, they used mosfets.

[J M Fahey]

On a "side note" of interest.....this amp runs 2x EL34.
The back panel says it is a 20 Watt amp.!
Have not been able to measure any voltages yet, so I have no idea, but I am wondering where that number comes from.?
Would Blackstar be running this amp at very low plate voltage for some reason.?
Well.......the reason would be to get power reduction. Guess what I am really wondering is.....what is their strategy with this amp...why the 2x El34 power section for a 20 Watt amp.?
Thank You

Cathode biased 2 x EL34 for >>>real<<< 20W RMS is an old and very classic British favorite in the Hi Fi area; while "20W", 2 x EL84 amps , no matter how you bias them, hardly have more than 15W RMS at clipping, if that much.

FWIW the very first Marshalls used large tubes (KT66, later EL34) , claimed 45W ... actually delivering close to 30W , again before clipping.

So maybe Blackstar chose to build the meanest loudest 20W rated amp in the market ... I wouldn't blame them at all

[g1]

Another thing, EL34's running at conservative levels will vastly outlive lower power tubes like 6BQ5's running at their limits.

[trem]

Cathode biased 2 x EL34 for >>>real<<< 20W RMS is an old and very classic British favorite in the Hi Fi area; while "20W", 2 x EL84 amps , no matter how you bias them, hardly have more than 15W RMS at clipping, if that much.

FWIW the very first Marshalls used large tubes (KT66, later EL34) , claimed 45W ... actually delivering close to 30W , again before clipping.

So maybe Blackstar chose to build the meanest loudest 20W rated amp in the market ... I wouldn't blame them at all

Oh Man...I did not even notice the 7W cat resistors, and by-pass cap.
I did see the giant bias (and balance) circuit for the power tube grids. So what is going on here.?
The power tubes are cat biased, but they also have a bias voltage applied to the grids, is that right.?
(i really have no idea what is going on in that bias circuit)
Thanks

Another thing, EL34's running at conservative levels will vastly outlive lower power tubes like 6BQ5's running at their limits.

Yeah...no doubt. Seems like, just about, every EL84 amp I see runs the crap out of the power tubes. I would think you are correct.? .....EL34 that are (i still can't check because of the PT) running at (maybe) 350 VDC on plates and a bit lower on screens might last a real long time.?

[trem]

An update.....just get a new PT from Blackstar.
On further inspection (once i got the old PT out of the chassis) I see one of the Red HT wires of the secondary was broken, almost all the way through. It is stranded wire, and there was One, small strand still connecting. The other strands may have been making intermittent contact. Most of the PT wires are folded up, and have a Zip-Tie or two on them. The break was in one of those folds, and under the plastic tie. Who knows, an assembler at the factory may have snipped the wire when he cut the excess off of the plastic tie.
Anyway.....now I know what was going on there.

On Page #3
D36 and D27
They are in parallel, reverse polarity, with a 100 Ohm resistor between them.
I am not really sure how a DMM works during the Diode Check Function.....I guess a small voltage is applied, and current is forced through the diode in the proper direction.?
If I twist 2 diodes together (reverse polarity) and put my meter (diode function) on them, i get 0.59V.
When I put my meter probes on D27 and D36, I get 0.054V
They read 98 Ohms with the meter set to Resistance.

That is my question, is that "normal".? If it were not so difficult, I would just lift a diode leg, but all these p[arts are soldered from below the board, and are hard to desolder. So I am not really sure what kind of reading I should be getting from D27 and D36. What should it be.?
Thank You

[g1]

That sounds normal. When you say the 100ohm is "between them" I assume you mean in parallel.
So you will always measure the low parallel resistance, even in diode check function.
If you check a 100ohm resistor on diode function, it will probably give you the same reading.

[trem]

That sounds normal. When you say the 100ohm is "between them" I assume you mean in parallel.
So you will always measure the low parallel resistance, even in diode check function.
If you check a 100ohm resistor on diode function, it will probably give you the same reading.

Communication via the keyboard is a skill (one of many) I do not have.....
Yeah...Sorry, that resistor is in parallel with those two diodes, and Physically it is also between the two.
At any rate, thanks for the info.
I will proceed.