Taming high plate voltages
I've got a late 1960's amp designed to run 7027A's and it runs very high plate voltages to get 75-80W.
I've seen a scan of a fax of a photocopy of the circuit diagram and there is a handwritten indication that the plate voltage when new was 575V. Measured recently my amp was 623V.
Modern 7027A's are 6L6's in disguise and 623V is way above max. spec. for any modern 6L6 I've seen.
Short of rewinding the OT, is there any other way to reduce the plate voltage?
Currently it is running a pair of JJ 7027A's that I fitted this year and it is running quite happily, although I have only been using it at low volumes at home at the moment.
whats the screen grid voltage
It sounds like it isn't just your plate voltages that are high. You need to lower the power supply voltage. If you had a variac or built a bucking transformer you could lower your mains voltage to what the amp is designed for. There are ways to lower voltages such as using zener diodes on the center tap (if you have one) of the high voltage winding of your power transformer but to drop 50V requires a big diode, a heat sink and a place to mount it. Probably not the best approach. The best alternative is to change the power transformer with one that provides the voltages that you need. This can get expensive.
If you knew the specs on the heater winding of the power transformer you could possibly swap in a set of kt88s and modify the bias circuit to run them. What kind of amp is it?
Fortunately, some of these tubes can take more than what the data sheets specify. The data sheets provides a spec for optimal service life. Some of the older Fender amps used to run higher plate voltages, they knew that they could get away with it. Like you said, the JJ's seem to be working out ok.
Is that measurement taken without valves in it? When loaded it drops a bit.
I'm wondering if part of the problem is that the mains voltage in New Zealand is (or at least used to be) 230VAC 50Hz. If the power transformer is set up for 220VAC, the internal voltages are going to be a bit higher.
I would be curious how high the heater voltages are. Rising line voltages will bring up voltages in a vintage tube amp. There are many schematics for transformer buckers, designed to reduce the line voltage several volts, to bring the AC down to what it was when the amp was designed. A few people make them for sale, but building one isn't difficult. I have no experience dealing with 220volts and I don't know what is available in NZ.
My B15 was designed for 115V. Up here in my area the voltage hovers around 126V. I feel that if the JJs are running properly then leave well enough alone. If you were constantly blowing outputs then a change might be required.
Remember that the heater draw of the 7027 is 0.9A. Just about any tube swap will increase that draw by at least 0.6A per tube and possibly as much as 1.5A per.
Here, run 700V HT on a quad of EL34's ( 6CA7 ), your amp has been working since the 60's i take it? Aspects like back emf suppression on the OT ( prevents socket flash over ) will be of most concern for elevated HT/screen. Heaters are much more robust than many imagine, apart from suspect ( modern ) manufactures. Internal valve flashover is a worry of course, when exceeding manufactures recommended supply voltages, called "abusing a part".
Tubes can actually suffer less in circuit demand with higher HT to obtain maximum power transfer to the load. Power squares with HT increase for a given load impedance.
Specifically for the EL34, they will NOT take the same abuse that a 6CA7 will take, the EL34 is a lighter-duty tube. The 6CA7 will take a LOT of voltage, such as in some older 1950s Hifi amplifiers, (Marantz comes to mind), but an EL34 in the same socket will often have its bias "crawl around" in an unstable manner. This is probably a heating problem, but a lower bias does not usually work out well in those units.
However, the actual voltage capability of a tube is pretty high. For a start, the tube needs to withstand double the normal plate voltage, plus 30% or so for transients. When one tube is fully "on" (pulling the transformer winding down close to ground), the output transformer swings the other tube plate to nearly double the plate voltage.
The transients are common when running into clipping.
We got a run of bad tubes once (EL34s, as a matter of fact), that had not been "flashed off" for voltage. That process is used to clean up any tiny bits of junk that are inside the tube, since those bits can cause arcing.
If the amp were clipped using the bad output tubes , they would fail with "fountains of white fire" inside the tube. An interesting sight, but not very useful.
We rigged up a limited energy source of 2500VAC or so, and applied that to all the tubes in that lot. You could see bits of junk in the tube getting zapped by sparks. Once there were no more sparks, the tube was good to go. None of them ever failed for arcing in the amp.
Tubes which have been properly "flashed off" like that will stand higher voltages without arcing than they can take for power dissipation.
Thanks for all your replies!
I've found the voltages as measured and I've mixed up a digit or two.
Screen voltage was measured at 423V
Plate voltage at 610V, so that is a little bit better than I thought.
I had not even thought about the lines voltage coming into the house.
re: running different tubes, a 1970's version of the amp with exactly the same schematic had a 2x 6550 100W option running at plate voltage of 680V. I presume the power supply and transformers must be different. There is currently not much info on the web about these amps.
The amp is a Jansen Bassman - renamed Jansen Bass in the 1970's when Fender told them to change the name.
Several techs in NZ have experienced success converting the 1970's Bass 75's to run 6550s, but I'm not considering it at this stage.
As several of you have pointed out the JJ's are currently handling it OK. I guess I need to try it out at higher sustained volumes to see how it works out.
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