I would have to go back to my original tag board design to do that.... Not keen. I'll see how it goes with a single resistor
As the feed for the EL84s comes directly from the rectifier B+ via the OP transformer CT, I would guess it won't be much under the 390V. I'll measure it and see.
Two parallel 1K 5W resistors in the B+ gives me 325V at the rectifier and 320V on the plate. I might need to add a 600 Ohm or so resistor into the bias, as with the new diode in place of the selenium the lowest it will adjust to is -13.1V. Not to sure what would be best, about 12.5V at a guess? Full volume with no input the amp is silent 🙂
You can use the 500Ω resistance now to measure the total current draw.
eg. If the HT off the rectifier is 390 volts and after the 500Ω it's 325v that is 65 volts drop. So 65v divided by 500r = 130mA. That is 10 ish watts on each EL84.
If you lower the bias, you will increase the total current, be careful not to exceed 150mA or 75volts drop. That is 12 watts per EL84 at 320 volts and 37.5 mA. Assuming the HT+ stays at 325 volts with the increased current. Also the 500r will be dissipating 11.25 watts... I will leave it to you to work out it exactly. Alan
eg. If the HT off the rectifier is 390 volts and after the 500Ω it's 325v that is 65 volts drop. So 65v divided by 500r = 130mA. That is 10 ish watts on each EL84.
If you lower the bias, you will increase the total current, be careful not to exceed 150mA or 75volts drop. That is 12 watts per EL84 at 320 volts and 37.5 mA. Assuming the HT+ stays at 325 volts with the increased current. Also the 500r will be dissipating 11.25 watts... I will leave it to you to work out it exactly. Alan
To be honest 10 Watts is plenty, and not working anything hard. It's in a 13ft x 13ft room, and at half volume it is plenty loud enough, also it sounds really nice, so I am tempted to leave well alone, apart from replacing the PS caps'..... When they finally arrive 🙂
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Aluminum one is a FW bridge rectifier. Single one may be a singe diode, likely used to provide the negative grid bias voltage, as a guess.
The Ali' rectifier was selenium, I stripped it out and rebuilt it with 4 UF5407 800V 3Amp Soft Recovery Ultrafast Rectifier Diodes. The bias diode was selenium too, I also replaced that with one of the above.
The things some folk hide.
I noticed a couple of scratches in the paint on the front panel bezel. It looked like brass underneath, so I went for it 😀
Why would anyone paint a beautiful piece of brass, when all it needed was a polish and lacquer?
I noticed a couple of scratches in the paint on the front panel bezel. It looked like brass underneath, so I went for it 😀
Why would anyone paint a beautiful piece of brass, when all it needed was a polish and lacquer?
Well, properly chuffed now. A friend Russ of Russ Audio found the matching tuner from an associate of his in Belgium. It's all packed up and on its way here. I'll post more images when it arrives and I open it up.
The schematic of your Frank pram 30 can be found here (no charge):
FRANK pram 30
Click on the schematic and than follow the instructions. A downloadlink will than be mailed to you.
FRANK pram 30
Click on the schematic and than follow the instructions. A downloadlink will than be mailed to you.
Many thanks for the reply. That entry in the radio museum is actually mine, the amplifier there is the one in this thread.
Hello !
Working on a FRANK Pram 30 that has a failed (shorted) selenium rectifier. Plan to replace that dead rectifier as you describe and get the plate voltage down to 320V with a 500 Ohm resistor. However, would you share how you replace the main filter multisection capa's ? Did you use twist mount devices ? There is one from the US but only rated at 350V. 350V seems a bit low taking new rectifier and increased line voltages into account. The one from Germany is 500V, but slightly larger dimensions. As you have gone through this, your comments and insights will be appreciated. Any change on pictures of the changes ?
Thanks in advance !
Working on a FRANK Pram 30 that has a failed (shorted) selenium rectifier. Plan to replace that dead rectifier as you describe and get the plate voltage down to 320V with a 500 Ohm resistor. However, would you share how you replace the main filter multisection capa's ? Did you use twist mount devices ? There is one from the US but only rated at 350V. 350V seems a bit low taking new rectifier and increased line voltages into account. The one from Germany is 500V, but slightly larger dimensions. As you have gone through this, your comments and insights will be appreciated. Any change on pictures of the changes ?
Thanks in advance !