interesting amp & interesting problem
I have a very strange amplifier I lifted out of an older (EIA datecodes indicate approx 1964) low-end hammond organ. It is a reverb & power amplifier with what appears to be a EF86 pentode input, 12ax7 tone control, 12dw7 reverb send/receive, and a 12ax7 phase splitter. Power stage is two 6bq5s running around 320-410 volts.
It sounds (sounded) great, it had a big gibbs reverb tank on it. Long story short, I believe the reverb tank was left off and the amp on for a while, because the separate OPT for the reverb channel was clearly open. I dug around, and ripped up a broken AA5 store PA (? it was also a strange device - 50c5 power though) for its OPT, which actually seamed to sound good and match up nicely (1957 hand wound, somewhat similar ratio also SE). However, there was a terrible buzz which I later determined was the result of a heater wire trapped accidentally under the OPT (oops--cramped space). After moving this wire back, I powered up the amp. It had been working fine, but with an annoying buzz. Now it began to make incredibly terrible feedback noises, and clearly was putting out a huge amount of power doing that. I caught it quick and killed it. Tapping on the EF86 while it was still powering down seemed to indicate it might be microphonic now, but it had worked fine two days before. Right now I am looking at replacing this tube, but I wanted to ask the forum some questions.
Why in the world do some tube manuals list EF86 and 6bq5 as interchangeable? The pinouts are different for sure, as is just about everything else. The only thing I can think is that standard EF86 wiring might strap 6bq5 into triode mode? Dunno.
Second, is it possible that another problem might be causing the gain of the EF86 to go so high that although it is normally a little microphonic, it becomes dangerously microphonic and starts oscillating? or is this just a case of a bad pre-amp tube and all my tinkerings have not caused this problem? I can post pictures if you all are interested.
Thanks for helping a newbie out on this forum, and I am interested to see what you think of this amps design. There is a lot going on in there that I am still trying to reverse engineer, including a possibly unused tube section or two. Thanks!
I have a very strange amplifier I lifted out of an older (EIA datecodes indicate approx 1964) low-end hammond organ. It is a reverb & power amplifier with what appears to be a EF86 pentode input, 12ax7 tone control, 12dw7 reverb send/receive, and a 12ax7 phase splitter. Power stage is two 6bq5s running around 320-410 volts.
It sounds (sounded) great, it had a big gibbs reverb tank on it. Long story short, I believe the reverb tank was left off and the amp on for a while, because the separate OPT for the reverb channel was clearly open. I dug around, and ripped up a broken AA5 store PA (? it was also a strange device - 50c5 power though) for its OPT, which actually seamed to sound good and match up nicely (1957 hand wound, somewhat similar ratio also SE). However, there was a terrible buzz which I later determined was the result of a heater wire trapped accidentally under the OPT (oops--cramped space). After moving this wire back, I powered up the amp. It had been working fine, but with an annoying buzz. Now it began to make incredibly terrible feedback noises, and clearly was putting out a huge amount of power doing that. I caught it quick and killed it. Tapping on the EF86 while it was still powering down seemed to indicate it might be microphonic now, but it had worked fine two days before. Right now I am looking at replacing this tube, but I wanted to ask the forum some questions.
Why in the world do some tube manuals list EF86 and 6bq5 as interchangeable? The pinouts are different for sure, as is just about everything else. The only thing I can think is that standard EF86 wiring might strap 6bq5 into triode mode? Dunno.
Second, is it possible that another problem might be causing the gain of the EF86 to go so high that although it is normally a little microphonic, it becomes dangerously microphonic and starts oscillating? or is this just a case of a bad pre-amp tube and all my tinkerings have not caused this problem? I can post pictures if you all are interested.
Thanks for helping a newbie out on this forum, and I am interested to see what you think of this amps design. There is a lot going on in there that I am still trying to reverse engineer, including a possibly unused tube section or two. Thanks!
Well, it wasn't the EF86 going microphonic - I resoldered some bad looking connections (the RCA pin connectors are wobbling resistors) and I moved some wires around. No more crazy feedback loop either. However, the 60/120 hz buzz is back. Additionally, there is a sound like a typewriter coming through very faintly. Moving the reverb level knob causes both of these sounds to increase. Any thoughts?
It wasn't uncommon in some old time circuits to run tubes in excess of design centre voltages. 6BQ5 /EL84 running higher than 350V in class AB will definitely shorten their lives. With todays tubes don't go over the voltage mark.
It sounds somewhere you have a heater cathode leakage and if tubes go microphonic, replacement is the only cure. The same problem can occur if the negative feedback loop from o/p stage to input stage cathode is disconnected.
Check voltages between cathodes and heaters before replacing a new tube.
If some manuals list the EF86/6BQ5 as interchangeable then certainly a big mix up. Noway.
richj
It sounds somewhere you have a heater cathode leakage and if tubes go microphonic, replacement is the only cure. The same problem can occur if the negative feedback loop from o/p stage to input stage cathode is disconnected.
Check voltages between cathodes and heaters before replacing a new tube.
If some manuals list the EF86/6BQ5 as interchangeable then certainly a big mix up. Noway.
richj
Probably need to replace the power supply caps if you have not already done so. Unit is 43 yrs old and those caps are living on borrowed time at best.
EF86 are available on eBay, some production from Svetlana (Winged C) in recent years as well. There is a US industrial type equivalent to the EF86 which escapes me at the moment.
EF86 are available on eBay, some production from Svetlana (Winged C) in recent years as well. There is a US industrial type equivalent to the EF86 which escapes me at the moment.
After resoldering some pin connections that were suspect, I fired her up again and got no microphonics. I am pretty sure that was just a wobbly cold solder joint or something along those lines. The electrolytics probably need replacement, but I wonder if the hum has to do with the new OPT--it's not exactly matched. It was for SE 50c5 into (I think) 4 ohms. This is the 12au7-like side of the 12dw7 driving about 8 ohms, so that's a fairly large mismatch. Could that cause hum? The reverb knob clearly affects the level of the buzz--i.e. all the way down, I get the leaky electrolytic hum I was used to. All the way up, it buzzes loudly. I have read about using a balance pot in SE circuits but I haven't seen good schematics. I can post pictures if you are interested.
The plot thickens...
I replaced the RCA cables from the reverb tank to the amp with the original circa 64 corroded ones. No hum (normal background hum as long as an instrument is plugged in. Cables from the 99c store really are not the best components.
There is still a grounding issue, the hum (no longer a buzz) can be cancelled entirely by touching the transformers or chassis.
the amp was making swishy-washy noises as well as quiet popping. I swapped the 12dw7 out for an old old stock (probably now lower gain) mullard 12ax7 (thus better matched to the reverb transformer). Very quiet except the previous grounding issues. I'll probably need to change some resistors eventually but it does work.
1) are there disadvantages to using a 12ax7 section over the 12au7 section of the dw? current? plate resistance is now ok because I put in a different reverb transformer (i assume)...
2) the amp is star grounded, ground lift does not help. How should I proceed? I grounded one end of my DMM (shitty) set it for AC volts and moved it around the chassis. Up to .3 AC volts repeatable in some places, most <.02.
3) does 4 preamp tubes make sense for an el84 amp that does little more than reverb and some kind of tone control? it seems excessive, and some of the wiring around the 12xx7s doesn't make as much sense as it should
I replaced the RCA cables from the reverb tank to the amp with the original circa 64 corroded ones. No hum (normal background hum as long as an instrument is plugged in. Cables from the 99c store really are not the best components.
There is still a grounding issue, the hum (no longer a buzz) can be cancelled entirely by touching the transformers or chassis.
the amp was making swishy-washy noises as well as quiet popping. I swapped the 12dw7 out for an old old stock (probably now lower gain) mullard 12ax7 (thus better matched to the reverb transformer). Very quiet except the previous grounding issues. I'll probably need to change some resistors eventually but it does work.
1) are there disadvantages to using a 12ax7 section over the 12au7 section of the dw? current? plate resistance is now ok because I put in a different reverb transformer (i assume)...
2) the amp is star grounded, ground lift does not help. How should I proceed? I grounded one end of my DMM (shitty) set it for AC volts and moved it around the chassis. Up to .3 AC volts repeatable in some places, most <.02.
3) does 4 preamp tubes make sense for an el84 amp that does little more than reverb and some kind of tone control? it seems excessive, and some of the wiring around the 12xx7s doesn't make as much sense as it should
The disadvantage of using a 12AX7 in the reverb is that it will probably make for a muddy sound.
I think it is possible that one or more of the 12AX7s originally present in the amp has been used to replace another tube, like a 12AT7 or a 12AZ7, or even a 12AY7. If I were you, I would try to locate an original schematic - you might be working from the wrong basepoint.
There are modern replacement 12DW7 available. If you get one, be sure how you are using it - some have the lo gain and hi gain sides reversed, so you have to rewire!
In many parts of US, the electricity company ground is useless - it just connects to the neutral. Hammering a big iron stake into the ground and connecting your ground wiring to that can solve a lot of problems, as long as you make sure that you still have 115V relative to that ground - if they are incorrectly feeding you from another phase, you might have as much as 235V coming in. Check out your supply.
I think it is possible that one or more of the 12AX7s originally present in the amp has been used to replace another tube, like a 12AT7 or a 12AZ7, or even a 12AY7. If I were you, I would try to locate an original schematic - you might be working from the wrong basepoint.
There are modern replacement 12DW7 available. If you get one, be sure how you are using it - some have the lo gain and hi gain sides reversed, so you have to rewire!
In many parts of US, the electricity company ground is useless - it just connects to the neutral. Hammering a big iron stake into the ground and connecting your ground wiring to that can solve a lot of problems, as long as you make sure that you still have 115V relative to that ground - if they are incorrectly feeding you from another phase, you might have as much as 235V coming in. Check out your supply.
While I remember to mention it, the 6267 is the US replacement type for the EF86 - you could also use a 6084 /E80F (but NOT an EF80 - that is a different animal).
Those Russian NOS EF86 types are actually very good value - they last well! Curiously, someone turned up a "made in GB" Mullard EF86 recently which appears to have a similar mesh screen to the late Russian types. You live and learn ...
Those Russian NOS EF86 types are actually very good value - they last well! Curiously, someone turned up a "made in GB" Mullard EF86 recently which appears to have a similar mesh screen to the late Russian types. You live and learn ...
I eventually managed to get the damn thing working...the EF86 wasn't microphonic, I had accidentlly connecting a tone control to the reverb return, creating mad feedback and emphasizing the slight microphonic tendancies of the tube. This amp had a LOT of outboard stuff connected by color-coded RCAs, which of course now all look brownish. It seems to work fine with a 12ax7 in the 12dw7 slot, of course the reverb transformer was swapped too so I think it's all good...no strange glows, sounds alright to me.
baxthorpe - in regards to your first post, the amp has tube types stamped in near the sockets.
it does specify 12dw7/7427 - 12ax7/ecc83 - ef86/6267 - 12ax7/ecc83 - (2x) 6bq5/7189 & good ole' EZ81. However, one of the axs must have been replaced a long time ago, everything else is rebranded mullard & phillips and one ax is clearly a GE.
it does specify 12dw7/7427 - 12ax7/ecc83 - ef86/6267 - 12ax7/ecc83 - (2x) 6bq5/7189 & good ole' EZ81. However, one of the axs must have been replaced a long time ago, everything else is rebranded mullard & phillips and one ax is clearly a GE.
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