Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pullshocks on March 29, 2026, 02:26:40 pm
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My project amp (6V6 single ended) does not have a center tap for the 6.3V circuit. I have the heaters and a Hoffman relay power supply connected to the 6.3 supply
Although I am not having hum issues, I want to wire up an artificial center tap, with 220R resistors as on page 275-6 of Merlin's Designing Valve Guitar Amplifiers.
He calls for 1/4 watt resistors. If I understand correctly, the low wattage is specified so the resistor would act as a fuse in the event of certain problems elsewhere in the amp.
The only 220R resistors I have on hand are 2 watt. How much of a gamble would it be to use these? With the cost of shipping these days, I would rather not do a parts order until I need some other stuff.
Thanks
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You can use any 1/2W resistors between 47Ω and 220Ω. You should also float the relay power supply. IOW, don't connect any part of it to chassis.
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Thank you Sluckey.
For temporary testing purposes I wired in the 2 watters.
I "measured" hum as shown below with a SLM.
I tried the artifical center tap connected to ground, connected to 6V6 pin 8, and unconnected. EDIT: SEE DIAGRAM POST #8 The hum was lowest with the artificial center tap unconnected. This was surprising, Merlin says there will be "almighty hum" with the center tap unconnected.
I guess maybe I should order a humdinger pot.
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I'm not sure I'm understanding your explanation. Are you connecting both 6V6 pin 2 and pin 7 to pin 8 via (2x) 220r?
Although in theory that would be fine, for me it creates too much of a mess around one of your 6V6 sockets. And, since many of us use a 1r resistor between pin 8 and ground to measure bias, it would really throw me off in terms of biasing. The currrent is low, but not negligible.
I'd ignore the use case of the 1/4 watt being a fusible link. Merlin is correct, but the fault it is protecting is rare IME.
The practical place to create your center tap is at the beginning of the heater circuit. See the diagram on Merlin's page right under the Artificial Centre Tap heading.
And while I don't automatically use a humdinger, they are my first step when there is excessive heater related hum; they work well.
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I'd ignore the use case of the 1/4 watt being a fusible link. Merlin is correct, but the fault it is protecting is rare IME.
I would not ignore that! I've seen quite a few Fender amps that had the 100Ω artificial center tap resistors burned and cracked open. If you use a 2 watt resistor there is a good chance that the high current that exploded the 1/2 watt resistors will likely destroy the OT primary as well.
So, what burned the resistors in the first place. The most logical cause is an internal tube short between pins 2 and 3. This internal short could be caused by a tube failure, but more likely the speaker has been disconnected (maybe someone stepped on a speaker cable). So, there's no sound. One of the first things the guitarist will do is start twisting knobs on the amp and guitar while playing power chords. Since the secondary is open circuit due to the unplugged speaker cable, the signal on the secondary cannot be dissipated and it will "fly back" to the OT primary. The OT has become a stepup transformer and produces a high voltage pulse due to the flyback. This high voltage pulse looks for a path to ground and the best choice for that path is to arc between pins 3 and 2. And so you end up with a damaged tube and socket and exploded 100Ω resistor. But if you used a 2 watt resistor, there's a good chance it may not burn up, and your OT is now in jeopardy. In this case, the 1/2 watt resistor acted as a fuse and protected the expensive OT.
This open secondary scenario is very common, especially with head and separate speaker cabs. Most amps provide a degree of protection by using a shorting speaker jack. But that only works if the speaker cable is disconnected from the amp. If the speaker cable becomes unplugged from the cab, you still have an open circuit on the secondary. Other amps use high voltage flyback diodes on the OT primary for protection.
Bottom line, do as you wish, but Merlin's advice is solid and the danger of using high wattage resistors is very real, and more common than you may think.
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Good point. I haven't seen this in the amps I've built where I typically use 1/2 W resistors for these, but I forgot about disconnecting the speaker!
It's true. Punters do what they do!
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... I have the heaters and a Hoffman relay power supply connected to the 6.3 supply ...
Maybe there's already a heater reference to ground via the relay stuff?
... I tried the artifical center tap connected to ground, connected to 6V6 pin 8, and unconnected. ...
What's the V AC to the chassis of each heater leg in each of the above 3 arrangements?
I assume the 6V6 is cathode biased? If so is the cathode resistor bypassed?
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I'd ignore the use case of the 1/4 watt being a fusible link. Merlin is correct, but the fault it is protecting is rare IME.
I would not ignore that! I've seen quite a few Fender amps that had the 100Ω artificial center tap resistors burned and cracked open. If you use a 2 watt resistor there is a good chance that the high current that exploded the 1/2 watt resistors will likely destroy the OT primary as well.
So, what burned the resistors in the first place. The most logical cause is an internal tube short between pins 2 and 3. This internal short could be caused by a tube failure, but more likely the speaker has been disconnected (maybe someone stepped on a speaker cable). So, there's no sound. One of the first things the guitarist will do is start twisting knobs on the amp and guitar while playing power chords. Since the secondary is open circuit due to the unplugged speaker cable, the signal on the secondary cannot be dissipated and it will "fly back" to the OT primary. The OT has become a stepup transformer and produces a high voltage pulse due to the flyback. This high voltage pulse looks for a path to ground and the best choice for that path is to arc between pins 3 and 2. And so you end up with a damaged tube and socket and exploded 100Ω resistor. But if you used a 2 watt resistor, there's a good chance it may not burn up, and your OT is now in jeopardy. In this case, the 1/2 watt resistor acted as a fuse and protected the expensive OT.
This open secondary scenario is very common, especially with head and separate speaker cabs. Most amps provide a degree of protection by using a shorting speaker jack. But that only works if the speaker cable is disconnected from the amp. If the speaker cable becomes unplugged from the cab, you still have an open circuit on the secondary. Other amps use high voltage flyback diodes on the OT primary for protection.
Bottom line, do as you wish, but Merlin's advice is solid and the danger of using high wattage resistors is very real, and more common than you may think.
Thanks very much for all the responses
OK, until I get some 1/2 watt resistors, I will be **really** careful to make sure the speaker is always plugged in
So if I want to try a humdinger, it would need to be a 1/2 watt trim pot, right?
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OK I took another look at the hum with different artificial center tap connection points, this time with 4 connection options. This time the lowest hum came from connecting the artificial center tap to ground.
In answer to the questions posed by pdF64:
1) Per Doug Hoffman's instructions, no part of the DC relay supply is connected to ground. I checked this with my DMM. However, the relay secondaries do have a ground connection.
2) See attached diagram for VAC to ground measurements
3) Yes the SE 6V6 is cathode biased, and does have a bypass capacitor.
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Adding a picture
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Adding another picture, annotated this time.
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...
1) Per Doug Hoffman's instructions, no part of the DC relay supply is connected to ground. I checked this with my DMM. However, the relay secondaries do have a ground connection. ...
Thanks for the info, what do you mean by "relay secondaries"?
A schematic would be a big help :icon_biggrin:
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...
1) Per Doug Hoffman's instructions, no part of the DC relay supply is connected to ground. I checked this with my DMM. However, the relay secondaries do have a ground connection. ...
Thanks for the info, what do you mean by "relay secondaries"?
A schematic would be a big help :icon_biggrin:
Thanks for taking a look at this.
Here is a picture showing the relay nomenclature. The picture shows a mechanical relay, but solid state relays are used in the amp. The principle and nomenclature are the same.
Also a partial schematic showing the channel switching relay secondary at the upper right. The complete amp schematic is not ready to share at this time. This channel switching arrangement was copied from the Tone King Imperial. The relay grounds one or the other of the rhythm and lead channels. That is what I mean by the secondary having a ground connection.
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Diagram added to clarify relay power supply wiring
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I hate it when I get inconsistent measurements or results, like the differing hum “measurements” in the reply #2 and reply #8.
“Measuring” hum with a sound level meter on my knee as seen in the photo in reply #2 is not a very precise way of doing it. There are at least 2 issues: inconsistent microphone position, and fluctuating readings on the SLM itself.
I rigged up a holder to keep the microphone in the same position for all readings, eliminating one source of error.
My Iphone spectrum analyzer shows a peak at 129 Hz that is quite steady (see picture below). Frequencies above and below the peak fluctuate considerably. Those fluctuations cause the SLM dBC readings to fluctuate, even though the hum itself is quite steady. That being the case, it would be a lot better to use a 120 hz octave band sound level meter.
With the SLM microphone held in a consistent position, I did another round of A.C.T. connection point hum testing. I was extra careful to eyeball the midpoint of the dBC fluctuations. I *think* the new readings are +/- .5 or .75 dBC.
First, I repeated some of the connection points I previously tried with the artificial center tap. This time I found the elevated heater connection points gave the lowest hum. Next, I tried the old AX84 style heater elevation, which has no artificial center tap. It was worse.
Finally, I added a humdinger circuit (see schematic below) and watched for changes as I rotated the pot. This would have gone a lot easier with an octave band SLM, but as far as I could tell with my current SLM, the humdinger gives at least 2 dBC lower hum than the artificial center tap connected at the same point. I was surprised at how far off center the humdinger needed to be adjusted to get the lowest hum.
The voltage divider gives an elevation of about 68 volts. I don’t know if this is an ideal elevation voltage or not. The hum readings at 18 volts elevation (6V6 cathode) were very close.
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There is a much simpler way to accurately measure hum at the speaker. Just connect your meter set to measure AC volts across the speaker wires. Very repeatable.
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There is a much simpler way to accurately measure hum at the speaker. Just connect your meter set to measure AC volts across the speaker wires. Very repeatable.
That's great! I assumed it would require some elaborate piece of specialty test equipment. Another case of not knowing what I don't know.
Excuse me while I peel the egg off my face....
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Pullsocks
You will only know, when you know ie have discovered and learnt :laugh:
...and that's why I recommend this forum, because people are eager to share and encourage those of lesser experience.
Coming from a Test and Measurement background (as well as ElectroAucoustic testing), I find a really reliable way of quick testing around a tube amp, is with my common variety Fluke 79 which has True RMS AC. Very accurate for DC/AC measurements on PS and as Sluckey said, any Hum (AC) on speaker O/P.
I have actually tested my Fluke 79 (with an Audio Precision APX525 Audio analyser) and can confirm that Fluke has managed a fairly even Frequency response 50Hz to 1kHz on AC Volts. Fantastic range for most tests on our Muso amps for a very reasonable price. I would imagine many of the cheaper True RMS MultiMeters would be very good too. So you can do a lot with a reliable Multimeter including using a 400Hz (if you have access to some form of signal generator).
Keep up the good work (and exploration) :wink:
Regards
Mirek