Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbaum on March 03, 2021, 10:34:07 am
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New builder here. I recently built a PR from a Mojotone kit. After a little troubleshooting to reduce hum, I have it up and working and it sounds great. However, I am wondering how much of the hum I have is normal vs. fixable.
I was reviewing the Mojotone wiring diagram vs. the Hoffman diagram and discovered a different in how V5 and V6 power tubes are grounded and I'm wondering what the possible affect, if any, might be? In the Mojotone diagram pin 1 of V5 and V6 is left open and pin 8 is grounded but in the Hoffman diagram pins 1 and 8 are linked and then grounded. I also looked at Rob Robinettte's PR diagram and he also has pins 1 and 8 linked and then grounded. (Images attached)
Thanks for your input.
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In a metal enclosed 6V6, pin number one connects to the metal case that serves as a shield, and should be grounded. In a glass enclosed 6v6, there is no shield, and pin number one does not connect to anything. However, there is no harm if pin number one is grounded. The caveat is that pin number one cannot be used as a tie point for other components such as a grid stopper.
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thanks 66STRAT - that is helpful and good to know.
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I think the higher level distinction is that some like RobRob and prob Hoffman like to add grid stoppers and/or screen resistors that were not included in the authentic PR schematic, model AA1164.
What they are doing is more akin to the AB763 Deluxe reverb. This includes the grid stopper and/or screen resistors.
Either approach works. Just pick one and stick with it. Follow the layout.
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Adding to what jordan86 said... The Fender AB763 amps use a third method of wiring the output sockets that allows the grid stoppers and screen resistors to both be mounted directly on the socket. Look at page two of this pdf...
http://sluckeyamps.com/phoenix/phoenix.pdf
I think the main reason you will see pins 1 and 8 connected together is because the EL34 has an additional "suppressor" grid that is connected to pin 1 and it is convenient to strap pins 1 and 8 together. This also allows idiots to run EL34s in their Princeton Reverb. :icon_biggrin:
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Hoffman puts the screen resistor on the main board, so I wouldn't try to follow that layout with the Mojo kit.
I built the Mojotone kit myself last May. I used RobRobinette's "mod" layout (which is identical to Sluckey's Phoenix and AB763 Deluxe Reverb). It's classic Fender. If you don't plan to use EL34's, I'd wire them this way as my first choice. See
https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Deluxe_Reverb_Layout_Signal_Flow.gif
https://robrobinette.com/images/Guitar/AA1164/AA1164_Princeton_Reverb_Layout_Mods.png
This probably does require you having the grid stopper and screen grid resistors on hand, as I don't think Mojotone includes them in their kit though. If you don't want to order parts, I'd just leave it wired it to the Mojotone layout/schematic and play away.
Per Sluckey, linking Pins 1 and 8 is probably not necessary at all when running 6V6, 5881, or 6L6, so don't sweat over them not being linked.
If you really want to make life easy. Replace the wire from Pin 8 to ground with a 1ohm, 1% resistor. One end to pin 8, one end to ground. Measure across that resistor in millivolts to get your milliamps of current. Multiply that by your plate voltage on Pin 3 to get your plate dissipation in watts. Should be somewhere around 18-23 milliamps (measured as millivolts).
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Thanks everyone. I think I’m going to adopt RobRobinette’s “mod” layout. Thanks for the feedback!
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jordan86, wondering if you can give me a little more direction on your last paragraph. Sorry for the noob questions, I'm learning a lot very quickly but that requires asking some stupid stuff at times.
I gather that you are talking about making it easy to set the proper bias? Could you run me through this process in a little more detail? I have added all of Rob's "mod" layout suggestions and it has made a big difference. Last thing I need to do is add the adjustable bias pot and set the bias but it's the first time I've ever done this so I could use a little guidance. Thanks for your help!
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I just replied to your other thread. That other thread is how you ADJUST the bias. These 1 ohm resistors simple help you MEASURE the bias. You should have a plain jumper wire going from Pin 8 to ground/chassis. Replace that wire with a 1 ohm resistor on each tube socket. So the resistor is now between Pin 8 and ground. When checking bias, set your multimeter to DC mv. Probe each side of that resistor. That value is your mA of current. Multiply that by the plate voltage on pin 3. That's your plate dissipation.
You should be seeing something in the ballpark of 400-440v DC on pin 3 of the 6v6's. And something in ballpark of 18-25 mv dc across the 1 ohm resistor. ie...430v x .022a = 9.46W. Roughly 67% of max plate dissipation, if you're using some modern 6V6 rated at 14W max.
You don't have to do the 1 ohm resistors either. It just makes it easier to check the bias. If you skip them, you can check it the "long way" and do more math. "Long way" is on this link.
https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
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Thank you, I just saw your responses. I'm going to install the 1ohm resistors and bias it tonight. I also found a few other resources that outline the same method you described. It looks pretty straight forward. Really appreciate the help. A LOT to learn!
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... In the Mojotone diagram pin 1 of V5 and V6 is left open and pin 8 is grounded but in the Hoffman diagram pins 1 and 8 are linked and then grounded. ...
... I think the main reason you will see pins 1 and 8 connected together is because the EL34 has an additional "suppressor" grid that is connected to pin 1 and it is convenient to strap pins 1 and 8 together. This also allows idiots to run EL34s in their Princeton Reverb. :icon_biggrin:
It was Sluckey & I (IIRC) that drew up the original "Hoffman style" layout for the Princeton Reverb a number of years ago.
While 66Strat is right about the pinout of the metal 6V6 (also applies to metal 6L6), the reason we chose to tie Pin 1 and Pin 8 is because that's necessary on an EL34. And it does no harm to a 6V6 or 6L6, as most leave Pin 1 unused and the few others tie Pin 1 to the metal shell. This latter bit was more a happy-accident (on my part, anyway).
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Thanks HotBluePlates, I’m learning a lot right now thanks to everyone’s willingness to share. Great forum!
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That's your plate dissipation.
Technically it is plate + screen current (dissipation) - measured at the cathode. There is a margin of safety built in measuring current this way because it is assumed there is appx 2 ma of current in the screen at idle.
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https://youtu.be/MmhIn0N22fA (https://youtu.be/MmhIn0N22fA) at the 1:20 mark shows the 1 ohm resistors installed in an amp built to the conventional Fender layout.
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However, I am wondering how much of the hum I have is normal vs. fixable.
Had the same doubts. I fixed some issues and mistakes then compared to a factory PR reissue. I stopped worrying and just play ;)
See my post about PR noise and hum.