Quick question - installing new Power Transformer.

[Stratz]

I had the same problem as Terry with the CT 40-18016 in a Mojo Fender Deluxe Reverb kit build.

The Mojo/Heyboer 761 (362/362v) read 448v at the plates biased to 19mA w/119v line voltage and GZ34 rectifier.

The "lower voltage" CT 40-18016 (330/330v) reads 453v at the plates biased to 19mA w/119v line voltage and GZ34 rectifier.

[valvetrain]

I had the same problem as Terry with the CT 40-18016 in a Mojo Fender Deluxe Reverb kit build.

The Mojo/Heyboer 761 (362/362v) read 448v at the plates biased to 19mA w/119v line voltage and GZ34 rectifier.

The "lower voltage" CT 40-18016 (330/330v) reads 453v at the plates biased to 19mA w/119v line voltage and GZ34 rectifier.

Wierd.... there really should be a reason that the circuit is not pulling down more than that...

what brand of tubes (all) are you using, and what is the plate current rating and transconductance value of the power tubes ?
or any other matching/rating info?
do you have a scope to be able to look at the crossover notch?
really trying to determine of 19mA is the correct bias setting...
the "book value" really only applies to tubes that are "perfect" in the eyes of the RCA folks 60 years ago...
today's tubes and 60 year old tubes rarely match
i find that with JJ's or EH tubes that i have to run the bias up to about 22.5 to get the notch where it needs to be.
that normally pulls the circuit down into the expected range...
I use the 40-18066 in virtually ALL of my 6V6 amps and never have the B+ go much over 410 when the amp is dialed in on the scope...

please let me know any info you can....

[Stratz]

Matched JJ6V6S which is a 14 watt valve. JJ GZ34. These valves are from the Mojo amp kit.

19mA @453v was 61% PD.
I'm now running at 22 mA (71%PD) which lowered the plate voltage to 440v.
22.5mA would be 73% PD
I like my Fender 6V6 amps biased hot but I'm already over 70% at this point.

Sorry to say that I know nothing about transconductance values.
The only scope I own is a Circuit Gear GCR-101 PC Based scope/signal generator but I have not tried to use it with an amplifier.
Hope this helps some
Rob

[valvetrain]

Matched JJ6V6S which is a 14 watt valve. JJ GZ34. These valves are from the Mojo amp kit.

19mA @453v was 61% PD.
I'm now running at 22 mA (71%PD) which lowered the plate voltage to 440v.
22.5mA would be 73% PD
I like my Fender 6V6 amps biased hot but I'm already over 70% at this point.

Sorry to say that I know nothing about transconductance values.
The only scope I own is a Circuit Gear GCR-101 PC Based scope/signal generator but I have not tried to use it with an amplifier.
Hope this helps some
Rob

Thanks Rob...
The JJ's usually bias really cold so 20 to 22mA is normally not an issue... (as compared to old RCAs. etc..)
taking a couple day of vacation, so I'll get back to you on Tuesday after i check a couple of things on my bench...

thanks,
Rick

[Tom Phillips]

I had the same problem as Terry with the CT 40-18016 in a Mojo Fender Deluxe Reverb kit build.
The Mojo/Heyboer 761 (362/362v) read 448v at the plates biased to 19mA w/119v line voltage and GZ34 rectifier.
The "lower voltage" CT 40-18016 (330/330v) reads 453v at the plates biased to 19mA w/119v line voltage and GZ34 rectifier.

As I noted earlier in this discussion, it doesn't make sense that the B+ would be 5 volts higher when the secondary voltage is 32 volts lower.
I think there must be an issue with the (362/362v) and (330/330v) stated specifications for one or both of those transformers. Those figures may just be the "published" specifications. To make a meaningful analysis we need to know the actual high voltage secondary winding voltage readings with the actual primary voltage present when the reading was taken.

[big_teee]

This 6v6 voltage approach seems to be backwards.
Instead of taking a too high voltage and loading it down within limits?
Why not start with a lower voltage, one that keeps all 6v6 tubes used within limits?
I won't use anymore 325-0-325 or higher voltage PTs for 6v6 builds.
If I was to do my plexi junior build again I would go with 280-300-0-280-300, with diodes.
After the fact I see one of these would have been more suitable to my build.
EDCOR - XPWR Series Tube Power Transformers
I'm done trying to use and adapt a vintage PT application to a project build.
Going to try to start with project PTs with the proper voltage parameters!
T
**I think this one would have been perfect with diodes, for my plexi jr, and you could power 6v6, 6l6, or el34.
http://www.edcorusa.com/xpwr064
285 x 1.414=403v

[Tom Phillips]

...I'm done trying to use and adapt a vintage PT application to a project build.
Going to try to start with project PTs with the proper voltage parameters!...

Right on! That is the proper engineering approach. It avoids the problem of vendors using dumbed down transformer specs in their catalogs such as "PT for a 2X 6V6 amp" or "PT for Fender Bassman" etc. Musical instrument amps are built with basic electronic parts that function based on the laws of physics not with special parts infused with proprietary secret rock & roll sauce. We would not buy a set of guitar strings if the package just said "Strings for a Fender Strat" so why buy a transformer without seeing a complete set of specifications and having assurance that the specs are trusted and verified?

[Stratz]

To make a meaningful analysis we need to know the actual high voltage secondary winding voltage readings with the actual primary voltage present when the reading was taken.

Just tell me where/what to measure and I'll give you the voltages Tom.
I'm obviously very new to amp building.
Thanks.
Rob.

[Stratz]

UPDATE:

Well, I feel quite embarrassed guys.
While measuring the plate voltage on my new Mojo AB763 Deluxe Reverb build over the past few days I was continually reading plate voltages at idle that were well out of range with the manufacturers specs.

When taking readings with both the Mojo/Heyboer 761 and Classic Tone 40-18016 PT's installed I was using a bias tool that I had recently purchased. It finally dawned on me to take some readings from pin 3 on the power tubes with my Fluke 87 (same as the bias tool does) and I saw a drop of almost 20 volts on the plates compared to my bias tool.

All along while I was swapping PT's, rectifier tubes, checking and re-checking my work it was the bias tool that was not calibrated correctly.
I've now set the cathode current to just under 70% and my plate voltage is 426-428 volts at 20 mA cathode current with ~118 line voltage.

I'm truly sorry if I've caused any members that have tried so much to help me resolve this "non issue" any grief.

I've since re-calibrated my bias tool with my Fluke 87 and all is well.

In my opinion from this experience I find that both the Mojo/Heyboer 761 and Classic Tone 40-18016 power transformers fall right in the range that they should be for the AB763 Deluxe Reverb circuit with modern line voltages. Perhaps a tiny bit over because of line voltage but certainly not enough to raise any concerns.

Thanks again to everybody for all your help.
Rob.

[Justin Thomas]

Takes guts to come back and acknowledge that... Thanks for that, and also the heads-up to watch out for calibrated parts.

Justin

[valvetrain]

This 6v6 voltage approach seems to be backwards.
Instead of taking a too high voltage and loading it down within limits?
Why not start with a lower voltage, one that keeps all 6v6 tubes used within limits?
I won't use anymore 325-0-325 or higher voltage PTs for 6v6 builds.
If I was to do my plexi junior build again I would go with 280-300-0-280-300, with diodes.
After the fact I see one of these would have been more suitable to my build.
EDCOR - XPWR Series Tube Power Transformers
I'm done trying to use and adapt a vintage PT application to a project build.
Going to try to start with project PTs with the proper voltage parameters!
T
**I think this one would have been perfect with diodes, for my plexi jr, and you could power 6v6, 6l6, or el34.
EDCOR - XPWR064
285 x 1.414=403v

Well dang... the real issue finally comes out...
these transformers are for DELUXE REVERB builds or repairs...
which means... a 5AR4, 2 6V6's and 6 (count 'em) preamp tubes of which 4 are 12AX7's and 2 are 12AT7's...
there are specific resistance values on the plates and cathodes preamp tubes in order to get the tubes to bias correctly within the circuit...
(which equates to overall current draw and final DC voltages of the circuit)
so if using the right transformer for the wrong amp (or vice versa) yields unpredictable results,
then that's probably because you did not do the required math before starting an untested build...

now that you have realized what your modified circuit actually draws and have come up with an adjustment to your initial voltage requirements it appears to me that your comments and recommendations now have value to the community here...

so... before bashing a full range of manufacturers for not building transformers that magically adapt to your circuit mods please change the thread to reflect reality...

the statement that "Hey Folks, the stock ClassicTone Deluxe Reverb transformers may not work real well for Plexi-6V6 circuits... however the 40-18028 on the low voltage taps may work fine..." would be a LOT more informative to the rest of the folks around here and be a LOT more palatable to the manufacturers.

BUT... then again, as i stated earlier... I'm the guy that has to provide support and also does this for a living...

so if offered... your apology for the unnecessary and undeserved bashing would be gratefully accepted....

Rick

[big_teee]

I didn't bash anyone, & I have nothing to apologize for.
We discuss things here, that is what we do.
I bought a PT for 6V6 circuits.
My mistake was buying a transformer for vintage amplifiers.
ClassicTone doesn't really sell any project transformers for 6v6 builds.
If even with a 5AR4 rectifier, 2-6V6s, 6 preamp tubes , it drops to 425v, that is still higher than most vintage 6v6 specs.
That is why I will try Edcor in the future, They have a lot of under 300v PT's, for projects.
If I'm building a project, Going to buy a project PT, not a Vintage Amp PT.
I recommend anyone that is building a 6V6 project, to do due diligence.
And, to not let the ClassicTone PT description, mislead you into a wrong purchase!
We are all entitled to list our experiences, and opinions here.
Good Luck
T
**I said this before, and I would like to reiterate that I do really like the Classictone OTs, and chokes.
I think they are good products at a good value.
I plan on purchasing the OTs and Chokes in the future, and I only had issues with the PTs.