Using 7189's in place of 6BQ5's
- Thread starter Lavane
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FlaCharlie
Super Member
Does / did Fender even make an amp that uses 6BQ5s?
Seems like most of their amps that are in this power range run 6V6s. I've seen schematics for more powerful ones that use 6L6GCs and 5881s, I think. I don't know that much about guitar amps and this is mostly a stereo site, so no Fenders.
FlaCharlie
Super Member
Are you sure? I just found a write-up on the Tremolux and it lists the outputs as 6L6s.
I then found a site with schematics and downloaded all the Tremolux, Vibrolux, Vibrasonic, and Vibroverbs. Most had 6V6s and 6L6GCs, one used 5881s. No 6BQ5s, though.
I had previously saved a lot of guitar schematics, many different brands, and I don't recall seeing a Fender with 6BQ5s. Anyone got a chassis number?
AmadeusMozart
Well-Known Member
Ya know, a funny thing: I modelled and tested the 6BQ5 and the 7868/7591 extensively and you know what? When done properly the least distortion is around the design centre level (in class A) with the screen just below the design centre voltage and the plate around 20V higher. (Karl A Spangenberg and O H Shade wrote about that., yes Sir.)
JohnT123
Super Member
You are correct. For one year only the Tremolux used 6BQ5 output tubes. I believe it was the first year piggyback design with blonde tolex. A very short lived experiment and rare as hens teeth.
Modern Fender uses the EL84 in the Blues Jr and Pro Jr plus a couple of short lived models.
Lavane
Lunatic Member
I thought I saw one sometime back with 6BQ5's.
FlaCharlie
Super Member
I looked at the operating points used by 7 different manufacturers in their PP 6BQ5 amps. Brands / models were (listed alphabetically): Grommes 40PG, HH Scott 299B, Leben 300, Packard Bell DPA-20, Pilot SA-232, Sherwood S-5000 and Voice of Music 1428.
Anyone care to guess (without researching) which amp runs their output tubes at the lowest dissipation and which runs the highest dissipation?
I'm surprised that nobody else wanted to take a guess at this . . .
Actually, it's the Sherwood S-5000 that has the lowest dissipation! And the Packard Bell console amp has the second lowest. The highest, by a fairly wide margin, is the Pilot SA-232.
Sherwood S-5000 -- 20.8mA -- 412.5v -- 8.58w
Packard Bell DPA20 -- 30.5mA -- 330v -- 10.06w
Voice of Music 1428 -- 41.7mA -- 275v -- 11.46w
Scott 299B -- 27.5mA -- 420v -- 11.55w
Leben 300 -- 40.8mA -- 290v -- 11.84w
Grommes 40pg -- 30mA -- 418v -- 12.54w
Pilot 232 -- 41.7mA -- 335v -- 13.96w
All of these are excellent amps to my ears, although I've never heard a Leben it also seems to get a lot of praise.
The Sherwood and the Scott I know are fixed bias, and the Pilot and the PB are cathode but I don't know the Grommes, Leben, or VM amps. Sort of curious how well that tracks with dissipation vs bias method.
Not sure why I was thinking the Sherwood worked the tubes hard. I guess I could have cheated and looked it up but thats just what rattled loose from somewhere.
Not sure why I was thinking the Sherwood worked the tubes hard. I guess I could have cheated and looked it up but thats just what rattled loose from somewhere.
I've built about 16 guitar amps that use EL84 (6BQ5) tubes and their equivalents, and worked on about another 10. With a few exceptions, they now run on 6P14-EV Russian tubes, because they handle more plate and screen voltage and current compared to 6BVQ5 tubes. I have a bunch of the latter as well as new production EL84 tubes, and they sound and perform differently from each other, though they do sound different, as judged by tube rolling within the same amplifiers.
Using original production 7189 tubes is a great option, as they are very high quality, but they are also very expensive now, although there are some being produced as 7189s that are supposed to handle more voltage and dissipation. However, in some amps wired to use only 7189A tubes, the socket must be rewired to use EL84/6BQ5 tubes, because Pin 1 on the 7189A is internally connected to Pin 2, whereas with 7189 and EL84/6BQ5 tubes, it is usually not. If the tube socket has Pin 1 connected to anything but the grid, you can have problems. For example, I restored an old Magnetone guitar amp that was wired for 7189A tubes, with the grid connection to Pin 1. Because those tubes are now rare and expensive I re-wired the sockets for current production EL84s.
The Russian tubes are indeed very rugged. I repaired one amp that was running almost 425Vp with plated dissipation at 15.5w. The sound was glorious, but the tubes were running incredibly hot, even though they were not showing obvious red-plating (with the room lights off, it was visible). It had been running like this for years for the previous owner, who found he could only use 6P14P-EV tubes in it without having to replace tubes every few weeks. I've got used tubes showing signs of long-time use or high plate dissipation that still perform well, even if they don't test as new.
Most of my amps don't push the tubes that high in voltage, but I do run the plate and screen current higher than with EL84s, with better tone and no ill effects in terms of tube life. Some are cathode-biased, others are fixed-bias, with the former being a bit easier on the tubes, even running the tubes at higher dissipation, as is typical for Class A cathode-biased amps. I bought up about 60 Russian tubes when they were on the tail end of being very available, but a couple of the small-production guitar amp makers bought up scads of those tubes, so the pickings are slim.
For amps to which I don't want to dedicate my remaining 6P14P-EV, I have experimented with almost all current production tubes, and several seem to perform well and sound good, albeit at lower Vp and plate dissipation than the Russian tubes. Some don't seem to last, and don't have good guitar amp tone to my ears.
Using original production 7189 tubes is a great option, as they are very high quality, but they are also very expensive now, although there are some being produced as 7189s that are supposed to handle more voltage and dissipation. However, in some amps wired to use only 7189A tubes, the socket must be rewired to use EL84/6BQ5 tubes, because Pin 1 on the 7189A is internally connected to Pin 2, whereas with 7189 and EL84/6BQ5 tubes, it is usually not. If the tube socket has Pin 1 connected to anything but the grid, you can have problems. For example, I restored an old Magnetone guitar amp that was wired for 7189A tubes, with the grid connection to Pin 1. Because those tubes are now rare and expensive I re-wired the sockets for current production EL84s.
The Russian tubes are indeed very rugged. I repaired one amp that was running almost 425Vp with plated dissipation at 15.5w. The sound was glorious, but the tubes were running incredibly hot, even though they were not showing obvious red-plating (with the room lights off, it was visible). It had been running like this for years for the previous owner, who found he could only use 6P14P-EV tubes in it without having to replace tubes every few weeks. I've got used tubes showing signs of long-time use or high plate dissipation that still perform well, even if they don't test as new.
Most of my amps don't push the tubes that high in voltage, but I do run the plate and screen current higher than with EL84s, with better tone and no ill effects in terms of tube life. Some are cathode-biased, others are fixed-bias, with the former being a bit easier on the tubes, even running the tubes at higher dissipation, as is typical for Class A cathode-biased amps. I bought up about 60 Russian tubes when they were on the tail end of being very available, but a couple of the small-production guitar amp makers bought up scads of those tubes, so the pickings are slim.
For amps to which I don't want to dedicate my remaining 6P14P-EV, I have experimented with almost all current production tubes, and several seem to perform well and sound good, albeit at lower Vp and plate dissipation than the Russian tubes. Some don't seem to last, and don't have good guitar amp tone to my ears.
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FlaCharlie
Super Member
The Grommes, Leben and VM are all cathode biased.
The Sherwood does seem to have a reputation for being tough on output tubes. If so, it's likely that the physical design plays a major role. In order to reduce the height of the cabinet they mount the tubes at an angle. They pack four output tubes and a 5AR4 rectifier into a very small area and then partially enclose them with a heat shield which serves to concentrate the heat further.
On the last one I restored I rewired the rectifier and output tube sockets with teflon insulated wire because the original wiring was looking quite toasty.
Lavane
Lunatic Member
I bought them because they were dirt cheap for a matched quad. The supplier I bought them from did a really good job of matching another quad of Russian power tubes I had to replace in a Russian made guitar amp. Honestly, I was just curious about the 6P14P's. They seem to hold up better in guitar amps from posts and threads I've read made by guitar players. I wouldn't use them in my ST-35 though. I can always make adjustments to the Magnavox if needed.
Most of the Russian datasheets I've seen that have been translated show 65mA but there are differently designated 6P14P tubes with somewhat different characteristics from each other, and can be helpful in distinguishing from EL84. Not sure why they would be "black box tubes", just reporting my own observations having used probably 100 of these tubes over 10 years or so. Guitar amps often run tubes at higher current and voltage levels than the datasheet specs, so failure rate tends to be higher than what is reported in audio, in my experience. Tubes that are ruggedized for military use and selected for longer life such as the -EV are pretty well supported with empirical evidence in the guitar amp community, though I really don't have any personal experience with them in audio. Can't imagine why they would not perform well, however.
Lavane
Lunatic Member
I ran across a list showing hours of use expected with the 6P14P types.
The 6P14P is good for 3000 hours. The EV's, EB's and K's vary from 5000 to 10,000 hours. All three are ruggedized to handle vibrations.
yeah the physical arrangement makes it tough. For some reason I was thinking they ran at high current though.
Depends how loud it gets run. I feel less bad about things being worked when doing something useful vs just burning up when sitting there. I tend to run things at relatively low volume so for me it wouldn't get worked all that hard. The Pilot 232 on the other hand would be cooking itself away whether I was listening loud or not.
true but the plate dissipation is an indicator of what the cathode is doing. Not running tubes at excessive pdiss would also reduce the abuse of the cathode.
lowering the heater supply can actually damage the cathode by not providing enough energy to boil off electrons properly. At that point it will start stripping the cathode and things fail. Not exceeding nominal voltage certainly helps but running them too far below nominal isn't good either. Small signal tubes don't care but output tubes don't like this.
cold cathode rectifiers exist, I've seen them in car radios. Strange things, but the current limits on them are also on the low side. All of the high current rectifiers are a typical "hot" tube, and the real high power ones get into fun stuff like mercury vapor.
lowering the heater supply can actually damage the cathode by not providing enough energy to boil off electrons properly. At that point it will start stripping the cathode and things fail. Not exceeding nominal voltage certainly helps but running them too far below nominal isn't good either. Small signal tubes don't care but output tubes don't like this.
cold cathode rectifiers exist, I've seen them in car radios. Strange things, but the current limits on them are also on the low side. All of the high current rectifiers are a typical "hot" tube, and the real high power ones get into fun stuff like mercury vapor.