Problem with excessive current draw

[Joe1968]

Hi,

Some days ago a friend brought his old bass & organ amp over for an overhaul. The amp is a head from the 70's (brand: Top Sound - nothing really valuable) which is basicly an AC30 circuit with 4 EL84 and a 2 channel input section.

The amp was working to a certain extent but the sound was poor and had really bad overall noise level (hum).

He want to use it for guitar and I convinced him to change the circuit closer to an AC30 TB. It looked like an relatively easy job, but it turned out to be a nightmare.

Here is what I've done so far.
1. Re-built the power section (new caps, power resistors, tube sockets, new rectifier diodes, new tubes, etc.). I've built the power section pretty much to specs from the Hoffman AC30 layout. The original was just using a cathode resister 84 Ohm instead of the 50 Ohm. One side of the filament was connected to ground. I changes this for 2 x 100 Ohms to ground (but also changed it back later during trouble shooting).
2. Rebuilt the pre-amp section to AC30 specs too (no tremolo).

So far so good.

When I turned it on first time w/o tubes everthing looked quite within the ballrange.
B+ around 310 VDC (filter caps higher without load of course), Heaters 6.9 VAC, etc. current limiter installed.

After I installed the power tubes (the old ones which have been working before - didn't want to risk the new glass) my current limiter was shining quite bright and the I had a serious voltage drop on B+. The Voltage drop over the cathode resistor was way over 15 Volts DC.

Double checking all connections and grounds didn't tell me anything new.

Then I tried a rather stupid thing by switching it on again without the current limiter. B+ was around 300 VDC, but I had a voltage reading at my cathode resistor of 50V (only for a couple of seconds!) Before I was able to switch it off, the cathode resistor was going up in smoke together with one of the 100 Ohm R's. My Variac unfortunately was at line voltage. Don't know what I was thinking.

Connecting the currernt limiter again. As a next step a swaped the cathode resistor for a 150 Ohm 10 Watt. But no difference. Still excessive current draw (of course not the same with the current limiter. The resistor was getting hot but didn't burn up). B+ dropping to 170 VDC. Bulb bright on when bringing the voltage up to 230 VAC.

I tried already with OT connected and w/o. Same result. The amp by the way is humming with the light bulb installed and AC voltage at line level!

Without the light bulb and AC line voltage reduced to 120 VAC. B+ is around 160 VDC, voltage drop over cathode resistor is about 15 VDC (with the 150 Ohm). Heater was around 3 VAC. When I tried to go up with the AC line voltage the thing is drawing way way too much current.

I douple checked the power section now over and over and couldn't see what I have done wrong. And I have done something wrong, because the amp was working with the same components (values) and circuit before.

I know it's something stupid I am missing here. But maybe there is someone out there who has some more ideas for me where to look at.

Any ideas are appreciated.

Best, Joe 

[eleventeen]

First, make sure you are not using / mixing up 7189's with 6BQ5/EL84, whether diagram relative to circuit or vice versa.

I don't consider AC30 work to be a walk in the park. That dude is very tight, very closely packed inside.

Any way to test your old & new tubes?

Any way to confirm whether you have bias on the output tube sockets? How is your bias being developed: tapped winding, pulled from one of the HV legs...? Is it there? Is the filter cap for the bias ckt installed correctly (eg; "backwards")

Tube or SS rectifier? To whack 300 volts down to 160, something got badly smacked.

New bad filter cap?

Shorted choke?

[Joe1968]

1. Tubes are all EL84.
2. Testest with old and new tubes
3. It is cathode biased. 84 Ohm to ground with 220 mf cap across.
4. Full wave retifier (diodes)
5. Filter caps seem to be o.k.

Joe

[VMS]

3. It is cathode biased. 84 Ohm to ground with 220 mf cap across.

Maybe you have a bad cathode bypass cap, like plexi50 had a while ago. Try the amp without the 220 mf cap.

Do you have any pictures?

[PRR]

> 7189's with 6BQ5/EL84

They are equivalent-enough, as near as any modern tubes ever are.

> voltage reading at my cathode resistor of 50V

I bet a penny you forgot to connect EL84 grid G1 resistors to ground. Power off, measure resistance from each EL84 grid G1 to common cathode point. Should be 100K-500K. Must not be infinity.

The other possibility is wicked oscillation. Put 2K resistor grid to ground at each EL84 and also in the driver/PI. That keeps the DC conditions essentially the same but kills the AC gain needed for amplification OR oscillation. If it now acts-nice, you need to figure how/where output is sneaking back into earlier stages.

[Joe1968]

Thanks
I checked the 0v reference on the power tube grids. You have been spot on. I missed a ground connection from the 220 k resistors.

No excessive current anymore. Installed the original cathode resistor (84 Ohm). Now I am a bit worried about the voltage readings.

1. 292 VDC after rectifier.
2. 1st main supply filter cap: 398 VDC (there have been two 47mfd 450V installed - I changed them for 500 V)
2. After choke: 396 VDC
2. pin 3: 12.7 VDC
3. pin 7 384 VDC
4. pin 9 394 VDC

I didn't take the voltages when the got here. Should have done it.

Could this be normal or do I still have a problem?

I didn't put in pre-amp tubes so far. But the power section seems to work. There is a hum coming off the speaker. Not that bad. Maybe I have to take care of it later.

Joe

[sluckey]

I think your voltages look normal. The low reading after the rectifier can be explained if the rectifier was not connected to a filter cap (maybe because the STBY switch was open for that reading). 400v may cause the PA tubes to have a shorter life but that's not uncommon in guitar amps.

[PRR]

> pin 3: 12.7 VDC
> pin 7 384 VDC

371V from plate to cathode.

This gets back to EL84 versus 7189. They are the same tube. A conservative design for a 12W tube with reasonable load impedance only needs about 300V, and the EL84 is rated 300V target, 330V absolute max. Later more-aggressive designs threw 400V at these tubes, the 7189 is rated to stand that safely.

The 1960 AC30 used about 300V and note: a 80 ohm common cathode resistor. In 1964 it changed to a 50 ohm resistor. That probably puts the EL84s over dissipation rating but what the hell. In those days amps got smashed before they got old. Also an amp like this, if you play LOUD it runs cooler.

In the 1960s, the 7189 was the safer bet. But tube production has changed radically. Philips could sort plate-metal into 300V, 400V, 600V stuff because they made millions of tubes (largely for 240V TV). Current production is in the tens of thousands (largely for over-volted gitar amps). AFAICT, most current EL84s will tolerate 400V and give useful life. When I overhauled a Traynor Bassmate, 410V on the plates, I stuck in Doug's Sovtek EL84s and in light duty they may go another 36 years.