Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MSVguitar on August 18, 2023, 01:26:16 pm
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I am modifying a 68 PRRI to move the output tube components off of the board to the tube sockets. There are 1N4003 diodes across a 1 ohm resistor from the cathode to ground. I don't know the function of the diode, I understand that the 1 ohm resistor is there to measure plate current. I'm thinking that I should just leave the diode and resistor out of the circuit and just ground the cathodes same as the original Princeton.
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The 1 Ohms are useful for reading bias current. Even 50 ohms won't affect the tube, which is several hundred ohms internally.
The diodes seem dubious. They can't come into play in normal use (that would need 600mA!) They may fit some fine-print fire regulation about burning the 1 Ohm resistors. Even so you can't easily burn a 1 Ohm 1 Watt part here, and a 1 Watt resistor in a tube amp is not a real fire. (A pound of burning tar on a transformer may clear a small room in a panic.)
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Thanks PRR for the quick feedback! I will leave the diodes out. Still undecided about the 1 ohm resistors. I haven't used those on my previous builds. I use the OT resistance method for bias current.
Regarding diodes for protection, there are also R3000 'Fly Back' diodes on the Output Transformer connection to the plate. Your thoughts on these would be appreciated.
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One other amp that has the diodes at the cathode + 1ohm resistor, power tubes are 6L6
http://www.prowessamplifiers.com/schematics/images/64%20Vibroverb%20Custom%20Schematic.pdf
Franco
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This presents a compelling case for the diodes https://www.thegearpage.net/board/index.php?threads/fender-64-custom-deluxe-reverb-handwired-schematic.1918269/page-2#post-26115004
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This presents a compelling case for the diodes https://www.thegearpage.net/board/index.php?threads/fender-64-custom-deluxe-reverb-handwired-schematic.1918269/page-2#post-26115004
Yes, that makes a compelling case for the diodes across the 1 ohm resistor. I don't enjoy wiring the filaments either and definitely wouldn't want to have to re-do it if a the 1 ohm resistor burned due to a shorted output tube. So, I think the best direction is to ground the cathodes. No 1 ohm resistor and no diode. I think another reason the diode was added on the 68 PRRI is that the 1 ohm resistor is mounted on the board, as well as the screen and grid resistors. If a tube shorted it would also burn up PC board. As I mentioned earlier, I am moving all of the output tube resistors and heater wires from the PC board and mounting them directly to the tube sockets to eliminate the potential damage to the PC board due to a high heat situation.
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This presents a compelling case for the diodes .........
I don't know how we ever survived the 1960s.
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Regarding diodes for protection, there are also R3000 'Fly Back' diodes on the Output Transformer connection to the plate. Your thoughts on these would be appreciated.
When a coil has current-flow, then shuts off that current, the coil's inductance causes a reverse-voltage spike in an attempt to sustain the current.
When you play your push-pull amp loud, it moves into the "Class B" part of its operation where one side shuts off while the other continues conducting. Those diodes at the OT-primary/tube-plate are there to prevent voltage spikes when that side of the power section shuts off.
Are they needed? Well, normally transformers are hi-pot tested to a few-thousand volts because the manufacturer knows they will experience those spikes. But some amp manufacturers (even back in the 60s and 70s) included those voltage-spike diodes in their amps. I dunno if the got a batch of bum transformers that prompted the addition, but it's cheap insurance and doesn't affect the amp's sound.
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Regarding diodes for protection, there are also R3000 'Fly Back' diodes on the Output Transformer connection to the plate. Your thoughts on these would be appreciated.
When a coil has current-flow, then shuts off that current, the coil's inductance causes a reverse-voltage spike in an attempt to sustain the current.
When you play your push-pull amp loud, it moves into the "Class B" part of its operation where one side shuts off while the other continues conducting. Those diodes at the OT-primary/tube-plate are there to prevent voltage spikes when that side of the power section shuts off.
Are they needed? Well, normally transformers are hi-pot tested to a few-thousand volts because the manufacturer knows they will experience those spikes. But some amp manufacturers (even back in the 60s and 70s) included those voltage-spike diodes in their amps. I dunno if the got a batch of bum transformers that prompted the addition, but it's cheap insurance and doesn't affect the amp's sound.
Oops, I overlooked that query.
I don’t like such diodes. Although they’ve got a really high voltage rating, if they do fail (which seems super rare but is very occasionally reported), silicon power diodes tend to fail short, which puts a big stress on things.
Also I find that even with my knackered hearing, they do affect the amp’s tone, or at least when it’s overdriven.
To check whether they affected the sound, I fitted a dpdt switch in their return path to circuit common, to enable instant AB comparison. They definitely acted to roll off some high frequencies. It was apparent on the scope too. This was on an AC30, and a 5E3 build.
I suppose that in some applications, the smoothed tone resulting from the diodes might be perceived to be a good thing.
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I'm not adding the diodes or the 1 ohm resistors.
Thank you to all for your feedback!
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The 1 Ohm resistors are a no brainer for making it simple to measure tube current and bias. The several tens of millivolts that they drop doesn’t have any impact on tube performance. There really is no down side.
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... it's cheap insurance and doesn't affect the amp's sound.
... I don’t like such diodes. ... they do affect the amp’s tone ... They definitely acted to roll off some high frequencies. It was apparent on the scope too. ...
Thanks for noting this!
Would that be capacitance across the diode junction, which then rolls off some high frequency since it's plate-to-ground?