Very low B+ in BF Vibrolux reverb

[slidincharlie (Carlo P)]

I have a friend's bf Vibrolux Reverb on my bench. It fried the rectifier, so I put a new JJ GZ34 in it. I also put a pair of small bottle TAD 6L6WGC. The manufacturer declares 500v plate voltage and 30w max. plate dissipation for these tubes, like a 'true' 6L6GC.
When I went to check voltages in this amp, I saw that it had a very low plate voltage: 360vdc (instead of 410v as per schematic) with the power tubes biased at 40 mA (which makes a cold 14.4w idle dissipation). The power tranny gives 300vac to the GZ34 instead of the expected 315.
This amp has got non-original power and output trannies.
With the previous Sovtek GZ34 thia amp had the same voltages.
This is an export model. If I put the mains selector to 220vac instead of 230, all voltages rise by about 10-15 v. But the mains here is 230v or more, so I believe I should keep the selector on 230v, correct?

My questions are: should I care about the low voltages (especially plate voltage on the power tubes)? May I bias the power tubes hotter (say 45-50 mA) or will I encur in possible damage? Should I check anything that may have caused the Sovtek GZ34 to fry, that can be linked to the low voltages issue?
As far as tone is concerned, the owner is a harp player and is very happy with this amp...

[Rick Erickson]

It's possible that a bad filter cap could be sucking some voltage. I would start by looking at the heater winding. Assuming parallel connected heaters - you should read 6.3V AC across the entire winding. (or across pins 2 & 7 of the power tubes) If it reads low you might be able to squeeze a few more volts out of the transformer by selecting a different primary tap. How much clean power does the amp put out? A Vib Rev is rated at 30W RMS as I recall. If the heaters read correctly I would then pull the rectifier tube and read the HT winding from chassis/center tap to either side. Then multiply that reading by 1.414 and you will get an idea of where the unloaded B+ voltage should be. If the customer is happy you might want to leave it alone and let him enjoy it as it sits. As for bias - yes you should be able to bump the current a little higher with less plate voltage - however doing so will likely drop the plate voltage even more.

OK - I just saw the line "This amp has got non-original power and output trannies."

That would be a big clue right there! The chances are quite good that the replacements aren't wound to the exact spec as the originals. If the customer is happy - leave it alone.

RE

[slidincharlie (Carlo P)]

Rick,
the heaters read about 6.25vac between pins 2 & 7 of power tubes.
The BFVR is rated 35w output, but I don't have a scope to monitor how much clean output it gives.

I am not sure I understood your instructions, please correct me if I did wrong:
with the rectifier pulled, the HT on each half of the secondary is 310vac (black probe of meter to ground). Actually it is 310vac on one end and 308 vac at the other end.
Multiplied by 1.414 gives 438. Is this the B+ voltage that I should expect with all the tubes in place?
Well, thye B+ as I said above is 360 vdc.
What's wrong here?
Thanks,

[milosch]

Have you checked for AC voltage (hum) on the B+ with the rectifier in place? Calculate the percentage vs. the DC voltage. I don't know, but it should be pretty low or non-existent. Is there audible hum on the output?

[slidincharlie (Carlo P)]

Do you mean checking for AC voltage on a B+ node with the amp on and standby switch in the playing position? I can read 6.5vac on the B+ node before the filters and on the power tube plates.
No audible hum though.

[Enzo]

AC x 1.414 gives the voltage the power supply would make WITHOUT a tube load. The number is the peak voltage of the AC, and so is the highest voltage the cap can charge to after the rectifier. Put the tubes in and it drops. 80v drop seems too much to me, so if the amp makes the 438 or close wihtout the tubes, then when it drops 80v with the tubes, either the tubes are drawing too much current or the power tranny is not powrful enough.

You have a non-standard power transformer. Just because the heater winding holds up doesn't mean the B+ winding can provide the current you want. If you get 310VAC without the tubes, then recheck and see what you get with the tubes. If your AC drops 50v, you know the DC will have to as well.

[slidincharlie (Carlo P)]

I’ll make a synthesis of voltage readings at the rectifier:
- without rectifier:
HT = 310vac (310*1.414=438)
- with rectifier and without power tubes:
HT = 310vac
B+ = 415vdc
- with rectifier and with power tubes:
HT = 300vac
B+ = 360vdc

Edit: voltage readings were very close with the previous power tubes (GT 6L6B), so I suppose that the current power tubes (TAD 6L6WGC) are not responsible for the huge voltage drop. Am I right?
And again: is there anything in my report that gives you guys a clue about the premature death of the previous Sovtek GZ34?