I frequently have Dynacord amps in repair. Anode voltages up to 800V.
Other old tube amps with 700+ Volts; no problem at all.
Ampeg SVTs, Orange 200W, and many others, all repaired and equiped with JJ tubes.
I think that I had to replace, maybe, 5 tubes over severall hundreds if not thousands.
Something I can not say of others. but as 6A3sUMMER wrote in post #20:
Sometimes, a particular manufacturer of tubes gets a bad name, because the tube was operated beyond its maximum ratings.
And over the years, some of the current manufacturers have improved their reliability.
But I'm very happy with JJ.
Other old tube amps with 700+ Volts; no problem at all.
Ampeg SVTs, Orange 200W, and many others, all repaired and equiped with JJ tubes.
I think that I had to replace, maybe, 5 tubes over severall hundreds if not thousands.
Something I can not say of others. but as 6A3sUMMER wrote in post #20:
Sometimes, a particular manufacturer of tubes gets a bad name, because the tube was operated beyond its maximum ratings.
And over the years, some of the current manufacturers have improved their reliability.
But I'm very happy with JJ.
And sometimes, bashing a manufacturer is the unknowledgeable persons way of pretending
knowledge, repeating others gossip. Just like bullying done by kids.
Experience is one thing. Gossip something else.
I have no problems with JJ tubes, in fact i see virtually no returns and this is by counting
sales in 4 digit volumes.
knowledge, repeating others gossip. Just like bullying done by kids.
Experience is one thing. Gossip something else.
I have no problems with JJ tubes, in fact i see virtually no returns and this is by counting
sales in 4 digit volumes.
I have had almost no issues with new production tubes from any manufacturers, including China.
As a contrast to that... I've had quite a few NOS power tubes fail catastrophically, even after reactivation. A 24-hour reactivation cuts the rate of thyratron behavior down considerably. As much as I love plasma, I'd rather it didn't take place in my amplifiers (except those with gas discharge tubes).
I think that if you buy the tubes that have been "burned in" so to speak, you're probably going to be fine. Usually if they're going to die young, they do so in the first couple hours.
As a contrast to that... I've had quite a few NOS power tubes fail catastrophically, even after reactivation. A 24-hour reactivation cuts the rate of thyratron behavior down considerably. As much as I love plasma, I'd rather it didn't take place in my amplifiers (except those with gas discharge tubes).
I think that if you buy the tubes that have been "burned in" so to speak, you're probably going to be fine. Usually if they're going to die young, they do so in the first couple hours.
Yes, that's the only way that makes sense to describe these tubes, as they doubtlessly also count five electrodes.
What about »real« pentodes for EL34's etc.?
OTOH, in the 1930ies here in Germany we had tubes that had originally been designed as pentodes, but got lost of their suppresor grids somewhen. Suppression in those tetrodes was achieved by another internal geometry, i.e. larger screen to plate distance. Remember e.g. EL11, ECL11 and many others.
Best regards!
> 600 volts across. Any problem in running either?
EL34 is actually rated 800V (which is reckless).
1956 rating on KT66 says 500V. I do not believe new-made KT66 are really much like Golden Age KT66, or that today's makers aim for extreme voltage UNLESS a large market of over-volted amps exists. Several classic amplifiers run EL34 at very high voltages. I have not seen a KT66 amp run much over 500V; indeed they tend to be 6L6GC-like designs, and I suspect today's "KT66" may be a lot of today's "6L6GC" parts inside. (I'd be agnostic about grid/beam until I did an autopsy.)
EL34 is actually rated 800V (which is reckless).
1956 rating on KT66 says 500V. I do not believe new-made KT66 are really much like Golden Age KT66, or that today's makers aim for extreme voltage UNLESS a large market of over-volted amps exists. Several classic amplifiers run EL34 at very high voltages. I have not seen a KT66 amp run much over 500V; indeed they tend to be 6L6GC-like designs, and I suspect today's "KT66" may be a lot of today's "6L6GC" parts inside. (I'd be agnostic about grid/beam until I did an autopsy.)
The life of a good tube often depends on the amp it is inserted into.
I have a pair of VTL amps that apparently were intended only for EL34 tubes.
But someone before me put 6550 tubes in the amps.
470k Ohm grid resistors, fixed adjustable bias, and the 6550 tubes
are a bad combination. It burned out the 10 Ohm cathode sense resistors.
Abuse a good tube and see what happens.
Who knows what else was damaged . . . that is a project for me if I get the time.
I am not a real fan of that amp, even with the right tubes in place. You have to unwire the amp just to get the circuit board out. And some of the parts and some of the traces are on the hidden back side of the circuit board.
As to JJ tubes, I have used many JJ preamp and JJ power tubes that I get from Eurotubes.com. I have not had a failure yet.
I have a pair of VTL amps that apparently were intended only for EL34 tubes.
But someone before me put 6550 tubes in the amps.
470k Ohm grid resistors, fixed adjustable bias, and the 6550 tubes
are a bad combination. It burned out the 10 Ohm cathode sense resistors.
Abuse a good tube and see what happens.
Who knows what else was damaged . . . that is a project for me if I get the time.
I am not a real fan of that amp, even with the right tubes in place. You have to unwire the amp just to get the circuit board out. And some of the parts and some of the traces are on the hidden back side of the circuit board.
As to JJ tubes, I have used many JJ preamp and JJ power tubes that I get from Eurotubes.com. I have not had a failure yet.
Last edited:
In the golden age of tubes there were many amplifier designs that ran on about 800 V plate supply. Good old EL34's of known provenience tolerated that. The issue was more with the sockets, which, when not of appropriate quality, tended to arc between pins 2 and 3. That arcing often infected the phenolic tube bases also.
Best regards!
That's why most tubes designed for those kinds of plate voltages bring that connection out to a top cap like they did on the 807.
Yes, of course! The pinouts of EL34 and all the 6L6 derivatives show exactly this Achilles' heel. E.g. the TT21 is a major improvement of the KT88 - same tube, but with a plate top cap.
Best regards!
It's also why most sweep tubes have a plate cap.
The reality is that with a reasonably clean vacuum, many of these tubes will hold off voltages far in excess of what they are rated for. The 829B, for example, can work quite well as a pulse modulator with plate voltages far in excess of the 750V it is rated for. Incidentally, there are two versions of this tube, one with mica spacers and one using ceramic parts instead. The ones with ceramic spacers will hold off 5kV (!), but apparently do not behave as well as a VHF oscillator. Supposedly the version with ceramic spacers was designated as the 3E29, but manufacturers don't all seem to have followed this. Regardless, this tube was regularly used to drive the 715 in radar modulators. The 715 in turn was used for pulsing the magnetron.
The reality is that with a reasonably clean vacuum, many of these tubes will hold off voltages far in excess of what they are rated for. The 829B, for example, can work quite well as a pulse modulator with plate voltages far in excess of the 750V it is rated for. Incidentally, there are two versions of this tube, one with mica spacers and one using ceramic parts instead. The ones with ceramic spacers will hold off 5kV (!), but apparently do not behave as well as a VHF oscillator. Supposedly the version with ceramic spacers was designated as the 3E29, but manufacturers don't all seem to have followed this. Regardless, this tube was regularly used to drive the 715 in radar modulators. The 715 in turn was used for pulsing the magnetron.
Hi many thanks, it appears to be a bias resistor failure as a replacement Gold Lion KT77 also failed, in for repair and hopefully just one Gold Lion needed to bias in.
Octals? Outside radio tranmitters?
Then you have led a more adventurous life than I. I can't remember working in or even seeing an octal worked over 625V.
Those were German musical instrument amplifiers, e.g. Dynacord Eminent II, Imperator II (a pair of EL34 at 750 V, Pout = 80 W) and others with the same output configuration, Dynacord Gigant and Gigant II (two pairs at 780 V, Pout = 160 W). They weren't adventurous at all, quite the opposite: With the right output tubes from well-respected German manufacturers (Valvo, Siemens, Telefunken, Lorenz) they ran very reliably on German dance hall stages in the 1960ies and 1970ies 
.
Best regards!
.Best regards!
Top Caps for RF Tubes, Yes it is very often required.
Consider a 2 - 30 MHz CW transmitter with a Class C output tube and a Pi network. These Pi networks have very High Q parts.
The Pi network matches impedances . . the correct plate load, versus the impedance at the coax that includes match or mismatch reflected from the antenna.
In Class C operation, an 807 control grid can go all the way to -200V. The 'definition' of that is 'extreme' cut off of the plate current. When the transmitter has to change frequency, the Pi network has to be re-tuned.
If during the tuning of the Pi network the two capacitors are mis-adjusted badly, then the Class C output tube plate voltage can be extremely high.
It may be 2X the B+, or 3X B+.
An 807 is rated for B+ of 750V. 2X that is 1500V, and 3X that is 2250V. The 807 can do this.
A tube that does not have a plate cap will not survive this, and the tube socket will not survive it either.
Most Hi Fi amplifier tube plates do not routinely go to 3X B+.
And most Hi Fi amplifier output tube control grids are not driven to -200V.
And there is the difference between requiring plate caps and not.
Consider a 2 - 30 MHz CW transmitter with a Class C output tube and a Pi network. These Pi networks have very High Q parts.
The Pi network matches impedances . . the correct plate load, versus the impedance at the coax that includes match or mismatch reflected from the antenna.
In Class C operation, an 807 control grid can go all the way to -200V. The 'definition' of that is 'extreme' cut off of the plate current. When the transmitter has to change frequency, the Pi network has to be re-tuned.
If during the tuning of the Pi network the two capacitors are mis-adjusted badly, then the Class C output tube plate voltage can be extremely high.
It may be 2X the B+, or 3X B+.
An 807 is rated for B+ of 750V. 2X that is 1500V, and 3X that is 2250V. The 807 can do this.
A tube that does not have a plate cap will not survive this, and the tube socket will not survive it either.
Most Hi Fi amplifier tube plates do not routinely go to 3X B+.
And most Hi Fi amplifier output tube control grids are not driven to -200V.
And there is the difference between requiring plate caps and not.
Last edited:
Usually it's an internal arc when it comes to 807s.
Correct me if I'm wrong, but can't we see these kinds of voltages when some idiot runs an amp into an open load?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.