Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fenderguy81 on June 03, 2021, 12:09:21 pm
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Hey guys, I have a 1965 Harmony H430 amp made by Valco and I am having trouble fixing a bad hum problem. The hum is constant with or without anything plugged in. It also does not seem to be affected by the pots. It has had all of the electrolytic caps replaced and all of the tubes have been tested and substituted to rule out a bad tube. I started pulling preamp tubes and the hum goes away when V5, the phase inverter tube, is pulled. The schematic shows a 12AX7, but this amp was wired up with a 6EU7 (chassis stamp indicates 6EU7) and wiring looks consistent with a 6EU7 pinout. I've been going through testing resistors and chopsticking around to no avail. Any recommendations would be greatly appreciated! Thanks!
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Correct schematic? https://el34world.com/charts/Schematics/files/Harmony/Harmony_430.pdf
Is it a hum or a buzz?
How about if the valve in V4 socket is removed? Or V3 etc?
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Is tubes's heater center tap to ground is good ? I had a amp with center tap is broken inside the Power transformer winding.
Are input jack's shorting contact to ground do their job ?
Did the amp have a 3 prong power cable with good ground ?
Good pictures of the amp may help.
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Correct schematic? https://el34world.com/charts/Schematics/files/Harmony/Harmony_430.pdf
Is it a hum or a buzz?
How about if the valve in V4 socket is removed? Or V3 etc?
I understand he do that with no success ;
"....I started pulling preamp tubes and the hum goes away when V5..."
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It had a hum before any work was done to the amp. A 3 prong cord with a dedicated ground to the chassis was added. It’s a hum, not buzz. Removing V4 does nothing. That is the only schematic to my knowledge. It’s the same except V5 is a stock 6EU7 opposed to a 12AX7.
The heater center tap is grounded along with the high voltage center tap. I haven’t thought to test the heater CT. Should I lift it from the ground connection and place 2 100 ohm resistors in its place or is there a better way to test it?
The jacks have some shielding that blocks them from view. I’ll remove/inspect the jacks to make sure that they are grounding properly.
Thanks!
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If the heater circuit has a low resistance to the chassis 0V, and the VAC on either leg are balanced, eg both measure 3.15VAC, then we can assume the heater circuit is fine.
A possible improvement could be to move to heater winding’s CT from the chassis 0V to the output valves’ cathodes, thereby providing a degree of DC elevation.
Does standby mute the hum (note it’s not a HT standby)?
Has a different 6EU7 been tried in V5?
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The first input jack on each channel is not the usual shorting jack type. The second input on each channel has shorted jacks so I used my burnishing tool to clean those and I tried to ground the first jacks to see if it reduced the noise, but it did not have any affect on the noise level.
I believe that the heaters are working correctly--low resistance/0V to chassis ground, and 2.9VAC on each heater tap.
Yes, the standby mutes the hum. When it is in standby mode, there is a small amount of noise that I would consider normal for a tube amp. The hum gets significantly louder when the standby switch is set to "play" mode.
I ordered a different 6EU7 from eBay and it did not change the noise level. Both tubes also tested good on my tube tester. I am still tempted to order a 6EU7 to 12AX7 tube socket adapter due to the readily available 12AX7 tubes and convenience.
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I’m not aware of any valve tester that assesses heater cathode leakage, a possible source of the hum here.
The vast majority of valve testers are a nonsense, really, at best a benign one, only useful function is testing for shorts.
What are the VDC on V5, each pin?
Same for the output valves V6&7, especially pin6 control grids.
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V5 (6EU7)
Pin 1: 0, pin 2: 0, pin 3: 0, pin 4: 1.1, pin 5: 0, pin 6: 134.6, pin 7: 174.4, pin 8: 0, pin 9: 1.59
V6 (6973)
Pin 1: 309.7, pin 2: .37, pin 3: .12, pin 4: 0, pin 5: 0, pin 6: .133, pin 7: 19.96, pin 8: 308.6, pin 9: 353.5
V7 (6973)
Pin 1: 308.4, pin 2: .270, pin 3: .071, pin 4: 0, pin 5: 0, pin 6: .074, pin 7: 0, pin 8: 306.8, pin 9: 353.4
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It had a hum before any work was done to the amp.
Answer: That is a must to know first
The heater center tap is grounded along with the high voltage center tap. I haven’t thought to test the heater CT. Should I lift it from the ground connection and place 2 100 ohm resistors in its place or is there a better way to test it?
Answer ; with all 3 wires disconnected , resistor between center tap and 2 other heater winding must be 0 ohms
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. Both tubes also tested good on my tube tester. I am still tempted to order a 6EU7 to 12AX7 tube socket adapter due to the readily available 12AX7 tubes and convenience.
Tubes testers can't "see" a noisy tubes. You must put know good tube.
Good pictures of the amp may help.
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The vast majority of valve testers are a nonsense, really, at best a benign one, only useful function is testing for shorts.
100% right
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The only 6EU7 to 12AX7 adapters I have found are on eBay and shipped from China. I think I’m going to rewire the tube socket for a 12AX7 to help eliminate the possibility of a bad 6EU7 tube. Any downside to doing so?
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A 'scope will help, don't you have one ?
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I think V7 pin 7 should be about 20V?
Apart from that, there’s no clues to the hum from the voltage survey.
If it’s not a massive PIA then rewiring V5 for a 12AX7 doesn’t have any obvious downsides.
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I rechecked pin 7 on V6 and V7 and got around 17 VDC. I do not have a scope, but am interested in getting one. Here are a few pictures after I rewired the 6EU7 to a 12AX7. Unfortunately, it did not help with the noise issue.
(https://live.staticflickr.com/65535/51230077539_ee52e68419_k.jpg)
(https://live.staticflickr.com/65535/51230077379_964c4b95a5_k.jpg)
(https://live.staticflickr.com/65535/51230077674_f12bc9f17e_k.jpg)
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Are you able to identify the frequency of the hum, ie mains freq, or twice mains freq?
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I think it’s a 60 cycle hum, but I’m not 100% sure of that. If there’s a way to check, please let me know.
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Maybe your meter has a freq measurement option on the VAC range?
You could research rigging up a scope using a soundcard and a suitable app.
Or look out some examples of 60 and 120Hz sine waves, and compare to what you’re getting from your amp.
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Hey, I just joined the forums - specifically so I could comment on this posting.
fenderguy81 - did you ever resolve this issue?
I have EXACTLY the same issue with EXACTLY the same model of Harmony. The OP's comments are eerily similar to mine - as in - identical. I, too, replaced all the E-caps and rewired the power cable to three prong specs.
Bumping this - any updates?
Thanks,
Fred G.
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Belatedly...... Why is the PT HV CT going to chassis instead of DIRECT to the main filter cap??
That puts a lot of rectifier ripple in several inches of chassis metal, which will spread all over the chassis.
Move the red/yel wire to the cap lug.
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I think it’s a 60 cycle hum, but I’m not 100% sure of that. If there’s a way to check, please let me know.
I know your question is a couple months old, but thought I'd share my solution for determining frequency that's cheap and really easy. I use an app (my phone is Android, don't know if it's available for iOS) called Spectroid. It's a frequency analyzer, and it works pretty well.
Hope you got your hum problem solved, BTW.
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Hey Medley,
I happened to come across this thread while Googling up this very identical problem, with this identical amp, so consider it "OPEN"! Thanks for participating!
fredg
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BTW - the hum in the one I'm working on is definitely 60 cycle. You can find videos on YouTube which can provide examples of each type. FWIW.
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I found that the guitar tuner app on my phone tells me the frequency of hum in an amp.
In case that helps.
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Sorry for the late response. I appreciate the advice about using an app to decipher the type of hum the amp has. I found a spectrum analyzer that showed a peak around 60 so I am going to assume it is a 60 cycle hum. Given that it is a 60 cycle hum, what would be the best way to go about trouble shooting the issue? I hope this also helps Fred G as well who is having a similar issue. Thanks!
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Belatedly...... Why is the PT HV CT going to chassis instead of DIRECT to the main filter cap??
That puts a lot of rectifier ripple in several inches of chassis metal, which will spread all over the chassis.
Move the red/yel wire to the cap lug.
^^^^^^^^^^^^^^^^^^^^^
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Wow! I don't know how I missed the previous post, but thank you very much! That got rid of the low frequency hum. I'm now chasing down a buzz. I guess it's more than one issue with this amp. Thanks again!
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Wow! I don't know how I missed the previous post, but thank you very much! That got rid of the low frequency hum. I'm now chasing down a buzz. I guess it's more than one issue with this amp. Thanks again!
Thank PRR. :icon_biggrin:
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Unfortunately, I tried that (relocating the PT HV CT) and it didn't help at all.
I DO, however, have a little bit different of a filter-cap placement strategy than the one pictured, and maybe THAT is causing the hum? I'll post a photo for comments a little later today.
I've ordered a set of matched power tubes and I'm hoping that might resolve the problem. I have had badly mismatched power tubes cause massive hum in a previous amp service, and it went completely away when I put in a matched set.
I'm also wondering if somehow I've made a mistake in adding a three-prong power cord and disabling the polarity switch? I've done so many of those without a hitch, but I guess there's always a first time...
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OK, as promised: These are the new filters in the "upper" chassis .
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"Bottom" chassis filters:
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How did you end up wiring your 3-way switch? I'd love to see and hear how you did it as this amp is a bit different than I'm used to. Do you still have the "reverse on" connected?
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How did you end up wiring your 3-way switch? I'd love to see and hear how you did it as this amp is a bit different than I'm used to. Do you still have the "reverse on" connected?
There is no reason to change the switch wiring. Just connect the power cord black and white conductors to the two middle switch contacts (doesn't matter which) and connect the green to chassis. Same as the current wiring except you add the green earth ground.
It would be a good idea to replace those two .02 caps with a Class X and Class Y cap. Use Class X across the PT primary and use the Class Y for the cap that connects to ground. I would also use a Class X for the .01 that is connected across the STBY switch.
Changing these caps will make the 3-way line reverse switch bulletproof. Having that switch can sometimes minimize/eliminate line noise and buzz.
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Welp...I pretty much disabled the polarity reversing "feature" of the switch, it is certainly easy enough to re-connect it back in, though. I've operated under the (perhaps mistaken) assumption that these are dangerous and aren't really necessary anymore. I can remember getting the p*ss shocked out of me back in the day using the line reverse on my '70's Twin Reverb, even with a three-prong plug. I haven't been shocked in a very long time, now that I'm using amps with "fixed" polarity (for lack of a better description)...
Installing a new set of matched power tubes didn't change anything at all with respect to that hum.
Any feedback on the way I set up those E-caps and potential hum-creating wiring? I'm certainly open to suggestions. It was just a real b*tch fitting them in there, so I took the risk of not being exactly theoretically perfect in my grounding scheme just to be able to get everything in. I've gotten away with this in the past in most of these oldies, but this one might be my "gotcha". You'll notice I grounded the 22uF and the 10uF ecaps to the nearest convenient ground position on the closest terminal strip (yeah, I know...). See the Bottom chassis 2 photo.
All comments/criticisms suggestions welcome - I need to get this amp fixed and back to the customer, and I'm pretty stumped.
Should I re-install the line reverse feature, then? I DO remember that these worked extremely well sometimes, "back in the day", when they didn't shock me senseless when I touched a microphone, lol.
What I did was to just snip off the jumper wires and reversed the black and white in order to have the 120VAC going through the fuse instead of the neutral doing so. The switch now has two "off" positions.
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Once you connect the earth ground to your amp chassis, your amp is safe whether it has a line reverse switch or not. However, you can still get shocked while holding the strings and touching another piece of equipment (such as a mic) that does not have a proper earth ground. Shocks can also occur due to improper house wiring. Once you decide to go "green" better hope all your bandmates do too! :l2:
What I said about Class X and Class Y applies anytime you have a cap connected across the line (Class X), or from either side, hot or neutral, to ground (Class Y). These caps are manufactured in a way that will not allow the cap to fail shorted.
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Any feedback (lol, no pun intended) on the way I grounded those two filters in the "Chassis Bottom 2" photo?
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Probably just as good as the original Harmony grounding. I don't like relying on a rivet for a ground connection.
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OK, I'm going to ask a dumb troubleshooting question:
Pulling the PI tube in this amp decreases the hum significantly. From this, am I to infer that the hum is IN the phase inverter circuit, or does this just tell me it is somewhere further upstream in the preamp?
Thanks for your patience!
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Could be the PI or anything prior to the PI.
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Ok, that's what I thought. Logically, then, pulling preamp tubes prior to the PI should isolate which section the hum lives in. If pulling any tube prior to the PI kills the hum, there's my suspect. If no effect, then the PI circuit is the perpetrator. This method would also tell me the issue doesn't come from the power section, correct?