Bad Cat Black Cat 30R Low B+

[vintagekiki]

I don't believe in such generalized statements.

Theory and practice have always been at odds
The enlarged Rk moves the operating point, the amplifier is no longer a "living fire", the sound is somehow compressed, compressed, so at least the musicians say ...
The solution for AC30 was the introduction of -Ug1, the tubes do not hot, and the musicians are habituated to the sound of class AB1 ...

[Helmholtz]

Speaking of which, I'm still confused about the leakage resistance subject. In post #9 you wrote "If an ecap shows a DCR (out of circuit - watch polarity) in the megohm range it should be ok for most applications." I'm having trouble squaring that with "So OL will indicate a DCR above 50M, which should mean a good cap".

My statement was about ecaps - not film caps. In my experience the leakage resistance of ecaps doesn't change much with applied voltage, so a faulty ecap typically shows with a DCR measurement. But as said, it's always safest to test leakage at the designated operating voltage.

Low quality or underdesigned film caps may develop a non-linear break-down effect (probably corona discharge - no pun intended) that results in strongly increased leakage at higher voltage. Such behaviour is not covered by standard cap theory. Often the leakage of film caps is induced by trapped moisture and varies with ambient humidity and temperature.

Also consider that a leakage resistance of 1M may be fine for a supply filter ecap, while a coupling cap might need to have a leakage resistance greater than 100M to not disturb tube operating conditions. Acceptable leakage depends on the application.

[Enzo]

Remember your meter tests cap at a volt or two. Caps can leak like a screen door at higher voltage and look pristine at a volt or two. Many old caps I would find a 450v cap is OK on my meter, bit starts to leak heavily over maybe 100v, and cannot get anywhere near 450v.

[Helmholtz]

Theory and practice have always been at odd

I'm a skilled guitar player and my ears are my most valuable test equipment .

[vintagekiki]

The topic has lost its thread (core)
What is actually happening with the Bad Cat BC30R.
Since there are no factory schematics, all voltages are to be assumed within the predicted limits.
The main question is whether the BC30R works or does not work.

[Helmholtz]

The topic has lost its thread (core)
What is actually happening with the Bad Cat BC30R.
Since there are no factory schematics, all voltages are to be assumed within the predicted limits.
The main question is whether the BC30R works or does not work.

See posts #81 through #84.

[tubeswell]

Is there a simple/practical way to check for leakage?

P56-57 in the attachment Darr4 Amp Signal Circuits.PDF

[bobloblaws]

P56-57 in the attachment Darr4 Amp Signal Circuits.PDF

Thanks tubeswell. The link is broken but I found it here: https://el34world.com/Forum/index.php?topic=21294.50 You get around :-)

I actually have a paperback copy of the Jack Darr book that someone gifted to me. I should study it more.

[Helmholtz]

P56-57 in the attachment Darr4 Amp Signal Circuits.PDF

His method (Fig. 4-4) looks quite familiar . Doesn't get any simpler.

[vintagekiki]

P56-57 in the attachment Darr4 Amp Signal Circuits.PDF

Invalid Page URL

[Dave H]

Invalid Page URL

Electric Guitar Amplifier Handbook - Jack Darr-56-57.pdf

[vintagekiki]

Thank you very much

[bobloblaws]

Well folks, the amp has a brand new rectifier tube, the existing one sputtered to it's ultimate demise the other day. I checked all the filter caps for leakage using the high voltage method, they all appeared to be fine and I checked all the coupling caps for DC leakage in circuit. I also tested all the preamp tubes and checked corresponding voltages.
B+ is still around 330V whereas I'm expecting 350-360V as previously discussed. I'm inclined to leave well enough alone for now as there doesn't seem to be anything out of the ordinary going on with the circuit. So unless anybody has other concerns I'll say that's a wrap (with the exception of the schematic that I'm still working on). I appreciate everyone's input and patience.

[Helmholtz]

Rectifier tubes of same type can vary in voltage drop.
BTW, what is your heater voltage?

[vintagekiki]

B+ is still around 330V whereas I'm expecting 350-360V as previously discussed.

Question
1. How much is the AC Main voltage.
2. On how much voltage is set up voltage selector on main transformer.
3. Is the main (first) elco 33 + 33u/ 450V new or "like new"

1)

Step by step
How much is voltage and ripple voltage at the +UA point (first elko 33uF / 450V) with and without 4 x EL84 output tubes?

[bobloblaws]

Rectifier tubes of same type can vary in voltage drop.
BTW, what is your heater voltage?

The new rectifier did not change the B+.

Heater voltage is 6.2VAC