Fender Brownface 6G6 - Tung Sol 6L6GC reissue tubes - plate voltage question

[earache]

Hello

I'm looking at an early 60's brown face Fender Bassman, 6G6 schematic.

Just put in some Tung Sol reissue 6L6GC STR power tubes. Dependent upon which tube is in which socket, I've measured from 528 to 601 V on the plates.

The data sheet states a 500V max for these tubes. Should I try to knock down the voltage, and if so, how? The amp sounds great as it is right now. These were alleged to be an "Apex Matched" pair of tubes from CEDIST.com, but they differ some 5 mA when measured with a Eurotubes bias probe (28.6 and 33.6mA)

http://www.dougstubes.com/6l6gstr-tungsol.pdf

[Enzo]

5ma sounds close to me, besides, you are assuming the rest of the amp is balanced so the only difference is the tubes, it seems.

600 volts sounds awfully high for an amp expecting 450 volts. Even considering that mains voltage is likely 10% higher than then. Is it the original power transformer and using the correct rectifier tube?

The voltage won't kill the tubes, but dissipation might.

[g1]

I'm very surprised that the small 5mA difference could account for a 70V difference at the plates.
The tube that measures 600V at the plate gives a proper idle current reading at the same time?

[Tom Phillips]

I'll chime in to agree with the above comments. Something is amiss with the amplifier or with the methodology or the meter used to take those voltage readings. I recommend that you concentrate on finding what's up with that before you proceed further with the retube.

[Mick Bailey]

Every time I get a strangely high voltage I check my meter. A low battery (in some meters) can give spurious readings. The tubes are a close enough match. Some of the old Fender drawings I've seen say match within 25%.

A high plate voltage is one thing, but in my view high screen voltage is more likely to kill a tube.

[pdf64]

When correct meter operation has been confirmed, check the mains and heater Vac.

[earache]

Hello All

Getting back to this amp after some other summer activities took precedence.

Enzo's comment has me checking to see if the transformers were changed. According to the 6G6 schematic, the power transformer should be 125P7A, but there is a 67233 in there. The output transformer wants to be 125A13A, but I find a 022871 in it's place. An internet search tells me that these are the same transformers but a newer designation (Fender Blonde / Blackface / Silverface Bassman Heads Style Transformers By ClassicTone)

Are these one-to-one replacements? Date codes on the transformers in question - Power 606140 Output 606713 These date codes seem to point to the output tranny being a replacement from '67

Next to check the voltages again with my DMM having fresh batteries installed.

[pdf64]

I think that the specs for Fender part # may change over the years, and that Brownface amps are reported to have rather higher HT than indicated on their schematic.

Having checked the mains and heater Vac, do the HT (eg at the standby switch) and then the plate and screen grid Vdc, all at idle.
If a plate voltage is much different to the HT, try the Vdc again with V4 removed.

[earache]

Well, my clairvoyant friends, you were correct in your assumptions that the meter was at fault. A new 9 volt battery later, and the voltages are in the 480V range.

I appreciate your help and am glad that I can lean on your experience!