Something's very wrong here. 😕 I would advise you to double check the values of your plate, cathode and feedback resistors, and also try using a different 12AT7. It's also possible the cathode cap(s) are leaky.
It's well worth getting right - the RH84 is a damned good little amp.😉
No need, its easily done particularly when your trying hard not to get zapped 😱
The good side is your amp may well sound even better If you can raise your 12AT7 plate voltages back up to design specs.
I'll remeasure my voltages later and post them if needed.
I did as you advised: checked the resistors (fine), checked the JJ 12AT7 (@ 80 in my Eico 666, so I replaced it with NOS Admiral @ 130-something)...
I also changed the values in the additional PI filter (1K, 0.47uF, 390, 0.1uF) to bring the B+ to almost-perfect 301 V...
This is what I have now:
EL84 plates: 290V/290V
EL84 cathodes: 7.56V/7.47V
12AT7 plates: 50.6V/62.6
12AT7 cathodes: 0.59V/ 0.56V
Heaters are now at perfect 6.3V
Do notice how, now that a strong tube is inserted into the circuit, 12AT7 plate voltages are even lower!
I followed the original schematic to the letter, except for the slightly changed PS; rechecked the wiring as well - seems like I haven't made any mistakes... The values of the components are spot on as well...
While waiting for someone more knowledgable to comment I thought about Mach1's suggestion of leaky cathode caps. Perhaps you got a bad batch or something.
I disconnected the ground lead from the cathode capacitor of one channel of my RH84 and it continued to work OK. If you try this you can rule out leaky cathode caps.
Both channels of your amp have similar voltage readings which suggests to me the problem might be of a more global nature.
Disconnecting the 10uf caps ground leads ( at the same time ) would also rule them out.
I tried this with no ill affect
Hope this helps 🙂
As per RH 84 - Tube Audio ...... RH DESIGN schematic there ISN'T a bypass cap on the 12AT7 cathode... And if you meant El84 bypass cap being leaky, I don't understand how that can make 12AT7 cathode voltage lower... Do you mind elaborating?
By the way: did you (soonerorlater & mach1) guys follow the above schematic 100% or with some variations?
Mea culpa. It's been a while since I built mine, which definitely does not have a bypass cap on the T7 cathode.If there were a cathode cap and it was leaky, it would cause more current to flow, thus pulling the plate and cathode voltages down.
This is a really difficult one. Given the checks you have made, I'm really at a loss to explain the low T7 voltages.
None on my 12AT7 either, only the EL84'S have them. I used my own power supply. My OT's may not be 5k, apart from that all is according to schemo ( I hope 🙄 )
It's not clear to me if your EL84's low cathode voltage is caused by a separate local problem ie leaky caps - not impossible but not very likely that both caps would leak and at the same rate imho - unless it was a batch problem. More likely to me is a global problem thats affecting your 12AT7 and EL84's alike.
I don't understand how your 12AT7 anodes are only seeing around 60 v when they are separated from a B+ of 300v only by a 22k resistor and a 10k resistor whereas mine are seeing 160v for apparently the same set-up.
All I can think of is maybe the 10uf cap between the 22k and the 10k is causing the difference. They themselves are only separated by 2 10k resistors ( 10k for each channel ). If one were to leak to ground it would have an effect on the other channel as well.
I'm totally prepared to retract some if not all of this by the way 😛
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Only fair I double check as well 🙂
Allowed my amp to warm up for an hour before I took any readings. No music was played so as not to interfere with the readings. All readings were in reference to the ground ie the negative lead of my meter was crocodile clipped to the earth rail ( a la Boywonder ) allowing me to put my hand back into the proverbial pocket. I prefer this method as I can wander about the circuit easily and quickly.
Starting at the B+ point common to both channels I got 295v
Roving to the 12AT7 side of the 10k resistor I got 254v ( 250v on channel 2 )
Moving on to the 12AT7 side of the 22k resistor I got 166v ( 154v on channel 2 )
This is my 12AT7 anode voltage (s) ( pin1/6 )
This point is common with the 0.22uf coupling cap so I took the opportunity to test the voltage on the EL84 side of the cap.
I got zero telling me that all's well and no dc is getting through to the control grid of the EL84 ( same on the other channel )
Moving across to the EL84 side of the 100k feedback resistor I got 278v ( 277v on channel 2 )
This point is common with pin 7 of the EL84 so is the EL84 anode voltage
Moving around to pin 9 I got 285v ( 282v on channel 2 )
This is my grid 2 voltage
Moving around to pin 3 I got 10.26v ( 10.37v on channel 2 )
This is my EL84 cathode Bias voltage.
Going back to pin 3 on the 12AT7 ( pin 8 on the other channel ) I got 1.27v ( 1.39v on channel 2 )
This is my 12AT7 cathode Bias voltage.
This would have been so much easier if I could have posted Alex's schemo and marked the values on 🙄
Anyone - Please feel free to correct any mistakes in my terminology
Okay, I’ll blame it on the Labour Day and the booze!.. Just kidding, the blame is on me. This thread is a proof that one must never stop checking and that in doing so one must never assume things. This thread is also a proof that communication/willingness to help/sharing of ideas 🙂this website) goes a long way. Huuge thanks to everybody and to Mach1 and Soonerorlater especially… Read on folks.
Basically, I printed out Soonerorlater’s post #29, went into the garage and started measuring voltages.
Some voltages were similar but some (again) were not… 12AT7 cathode bias half as much, EL84 grid 2 voltage quite a bit lower, 12AT7 anode voltage higher… Higher??? But didn’t I measure it before at 52V/62V? What’s going on? Pins 1 and 6, yes… But I just measured it “on the 12AT7 side of the 22K resistor”… What?? OH MY… WHATTHE*&^!!!!
I will NEVER again do anything this important while hungover!
I DID NOT CONNECT 22K TO 12AT7 PLATES!!!??? AND I DID MANAGE TO CONNECT 2.2K TO EL84 GRID 2 TO 22K (to B+ meant for 12AT7)!!!???
It’s a miracle that I didn’t fry anything… And it’s a miracle that it worked. Yes, the amp kept on playing and playing…
Oh my… I swear to God I liked the sound. Yes, it was different from the build with Hammond OPTs, but I attributed the difference to Transcendar OPTs: detail, harmonious, shimmering highs, not-one-note lows… Blissful. More of everything when compared to Hammond OPTs... And now it turns out it was not the new OPTs, it was the different circuit than originally intended… Mistakenly at that…
Basically, I printed out Soonerorlater’s post #29, went into the garage and started measuring voltages.
Some voltages were similar but some (again) were not… 12AT7 cathode bias half as much, EL84 grid 2 voltage quite a bit lower, 12AT7 anode voltage higher… Higher??? But didn’t I measure it before at 52V/62V? What’s going on? Pins 1 and 6, yes… But I just measured it “on the 12AT7 side of the 22K resistor”… What?? OH MY… WHATTHE*&^!!!!
I will NEVER again do anything this important while hungover!
I DID NOT CONNECT 22K TO 12AT7 PLATES!!!??? AND I DID MANAGE TO CONNECT 2.2K TO EL84 GRID 2 TO 22K (to B+ meant for 12AT7)!!!???
It’s a miracle that I didn’t fry anything… And it’s a miracle that it worked. Yes, the amp kept on playing and playing…
Oh my… I swear to God I liked the sound. Yes, it was different from the build with Hammond OPTs, but I attributed the difference to Transcendar OPTs: detail, harmonious, shimmering highs, not-one-note lows… Blissful. More of everything when compared to Hammond OPTs... And now it turns out it was not the new OPTs, it was the different circuit than originally intended… Mistakenly at that…
Back to RH84. Everything’s as it should be now. Checked the voltages again, compared them to Soonerorlater’s – almost the same. Fine. Listening. Much louder now, lows especially. Transcendars do go lower and with considerably more definition than the small Hammond 125BSEs…
Glad you seem to have worked everything out! As far as stupid mistakes go, I feel your pain. When I first fired up my '84 it sounded HORRIBLY distorted. A quick look under the chassis, and I spotted the problem: I had forgotten to solder those resistors leading from the B+ to the screens. Doh! 
I certainly hope you're enjoying your '84 as much as I am mine. It sounds great even though a pair of Hammond 125ESE's. I'm tempted to spend the big bucks and get a pair of Lundahls. I'll bet they'd sound incredible!
I certainly hope you're enjoying your '84 as much as I am mine. It sounds great even though a pair of Hammond 125ESE's. I'm tempted to spend the big bucks and get a pair of Lundahls. I'll bet they'd sound incredible!
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Hi FullrangeSR,
glad things worked out, I calculated your original power output ( correctly I hope 🙄 ) as 8 watts, compared to my 11watts with my setup. Changed my power transformer to a Danbury giving me a B+ of 340v.
I now calculate my power output as 14 watts
There is a lovely faint blue glow from the centre of my EL84's 😎
glad things worked out, I calculated your original power output ( correctly I hope 🙄 ) as 8 watts, compared to my 11watts with my setup. Changed my power transformer to a Danbury giving me a B+ of 340v.
I now calculate my power output as 14 watts
There is a lovely faint blue glow from the centre of my EL84's 😎
These values seem a bit optimistic to me for a single-ended amp so I'm away to read up a bit more about power equations.😀
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