• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Potentiometer sound quality

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
That is why I wrote "usually". However, 'my' suggestion may work for some other reader who may have a differently designed circuit...

To go along with this: with a 12AX7 running flat out, even a very crude DC heater (1 single diode, 1000uF, 1ohms, 1000uF) reduces the hum to a level that's waaaay below the hiss. I did it on my Fender Champ guitar amp, much less work than a rewire.

Yeah on V4 (12AX7), V5 (12AX7) on line stage and then AC on V6 (6DJ8) last line stage valve.

same on the phono stage, but not using that atm

Hum from the line stage alone means BIG trouble. Beg, borrow or steal a scope.
 
I've had SP8's and SP10's before, and hum has never been a problem.

In order to find what's wrong with yours, download the schematic, highlight the parts you've replaced, and post it here. Also, it would help if you had some test equipment, an oscilloscope would be ideal.

Meanwhile, here's a check list:
  1. Disconnect the preamp all together. Any hum only from the power amp?
  2. Reconnect the preamp, but disconnect all the inputs. Is the hum still present? Phono? Line?
  3. Check if your preamp and your amp have a grounded AC plug for a possible ground loop

Thanks, so my pre amp had hum and I took it to a repair guy. The hum was an old very mechanically noisy transformer that was replaced. It gives correct voltages now, it did not earlier. It came back working but still with hum. he replaced the following parts.

R7 300k resistor open cct RIAA stage anode resistor
2N5462 FETS replaced x 4
Mute cct cap & resistor replaced
C16 capacitor replaced v3/6 heater smoothing capacitor
ZD17 replaced power supply
R87 Replaced ,v8 anode resistor
ECC88 / 6DJ8 X 2 replaced
Transformer mounting plate modified, transformer fitted
Transformer phasing check with scope.

A second repair guy (first guy gave up....) dressed the DC heater cable which was causing hum, and then swapped a valve too. This helped but left residual hum.

I replaced 2 of the FETs (Q13) as the first repair guy put the wrong ones in. This was causing loads on hiss on 1 channel. This was with magnificent detective work/help from this forum - THANKYOU :) Voltages all correct now. One had turned into a 250ohm resistor.......

other parts replaced to reduce hum, made no change are power supply caps (C14, C15, C12, C35), and the auto-mute optocouplers Q13 are now out of circuit, which reduced hum a tiny amount. Resulted in a welcome improvement in SQ actually.

any cheap way to get an oscilloscope! any apps? I have a DMM of course.
 

Attachments

  • Screenshot 2020-12-03 at 13.49.06.jpg
    Screenshot 2020-12-03 at 13.49.06.jpg
    564.2 KB · Views: 318
  • Screenshot 2020-12-03 at 13.49.20.jpg
    Screenshot 2020-12-03 at 13.49.20.jpg
    635.4 KB · Views: 315
might be related, but if I touch the glass of V4 it hums like crazy, and if I then touch the chassis again it reduces.

I tried a can/earthed not earthed on it and it made no difference unfortunately - might recheck this as this actually makes little sense when I thin about it now
 
Meanwhile, here's a check list:
  1. Disconnect the preamp all together. Any hum only from the power amp?
  2. Reconnect the preamp, but disconnect all the inputs. Is the hum still present? Phono? Line?
  3. Check if your preamp and your amp have a grounded AC plug for a possible ground loop
[/QUOTE]

no preamp virtually no hum in the power amp (valve 211)
hum irrespective of input and with zero gain still hear it as you switch mute on/off
both have grounded AC plugs
 
Thanks, so my pre amp had hum and I took it to a repair guy...
A second repair guy (first guy gave up....) ...
any cheap way to get an oscilloscope! any apps? I have a DMM of course.

Ouch!!!
Sorry to hear about your Odyssey; must have costed you a grand, right?
Reminds me of a guy who posted a similar ripoff with his Threshold preamp, over at the PASS section I think.

Regarding scopes, it depends on where you're located, and how brave you are. I have a Tektronix 100MHz analog scope I paid about $100 on eBay; it wasn't really working, but it way an easy fix with half a can of contact cleaner. Before that, I had a Trio 30MHz I sold for $50 to make room for the Tek. So that's the ballpark.

Alternatively, there're many digital scopes on eBay and Aliexpress that look attractive, but I have no first hand experience with them. Hopefully somebody will chime in. In any case, buy a couple of 10x probes to deal with tubes.

no preamp virtually no hum in the power amp (valve 211)
hum irrespective of input and with zero gain still hear it as you switch mute on/off
both have grounded AC plugs

Possible ground loop.
My suggested procedure (don't tell anyone):
  1. Buy a life insurance, make me the beneficiary
  2. Temporarily remove the ground wire of the power amp
  3. If that fixes the prob, add a ground loop breaker in the power amp, and put the ground wire back

Is the Alps Blue Velvet pot considered as good quality?
It's widely used, even in the ARC gears, but considered only OK-ish by the top guys. TKD's are supposed to be better
 
Last edited:
Ouch!!!
Sorry to hear about your Odyssey; must have costed you a grand, right?
Reminds me of a guy who posted a similar ripoff with his Threshold preamp, over at the PASS section I think.

Regarding scopes, it depends on where you're located, and how brave you are. I have a Tektronix 100MHz analog scope I paid about $100 on eBay; it wasn't really working, but it way an easy fix with half a can of contact cleaner. Before that, I had a Trio 30MHz I sold for $50 to make room for the Tek. So that's the ballpark.

Alternatively, there're many digital scopes on eBay and Aliexpress that look attractive, but I have no first hand experience with them. Hopefully somebody will chime in. In any case, buy a couple of 10x probes to deal with tubes.



Possible ground loop.
My suggested procedure (don't tell anyone):
  1. Buy a life insurance, make me the beneficiary
  2. Temporarily remove the ground wire of the power amp
  3. If that fixes the prob, add a ground loop breaker in the power amp, and put the ground wire back


It's widely used, even in the ARC gears, but considered only OK-ish by the top guys. TKD's are supposed to be better

I have a TKD now to swap from my old scratchy 35 yr old ALPS and I would say it does sound better now the damn caps are running in :)

Tried the ground out already on power amp no change :-(

cost me £500 for the work, I did get a new transformer installed I suppose.....found a fantastic supplier in the UK who made it to specification supplied from the ARC circuit for about £90 delivered. I was thinking it might not sound like ARC spec product which was an absurd price for parts. I think it sounded better but hey it definitely did not wound worse :) Transformer Equipment Ltd. | Sandwich Kent all voltages spot on.
 
Not only capacitors.
My Gustard AK4499 DAC sounds noticeably better after a full week of breaking-in.
No big deal, play of loop of music, switch off the power amp, and come back later. Here's how I break my PS caps. Eric Clapton’s No Reason to Cry works great :)
898841d1606992301-potentiometer-sound-quality-caps-burn-jpg


Mmmhhh, I see a mixed bag here.

Good ideas, mixed with clearly visible problems. :rolleyes:

Good: those crocodile clips are maturing fine.

Not a single Green one visible.

Half are going through the yellow stage, fine.
They are developing flavour.

Half have reached mature Red sweetness, harvest them NOW, do not freeze them (Cryotreatment makes them stringy and bitter) , worst case keep them in the fridge for no more than 2 or 3 days.
My Family loves them, they disappear within hours :D

BAD :mad: :
You let 2 of them untouched for too long, now they turned BLACK!!!
Do not even TRY to taste them, they will give you bellyache.

Do not even feed them to your pets, they are UNSAFE by now.
Wrap them in aluminum foil , even better in lead foil in case they turned radioactive, and dispose of them in a safe manner.

GOOD: congratulations on your idea of wrapping caps tightly together with rubber bands, that way the breaking in marginal field does not go to waste, but preconditions those nearby.

This thread should be a stickie, chock full of useful Audio info.

PS: Eric Clapton´s Layla works even better, because it was recorded on a Fender Champ.
Gobs of single ended Tubyness.
 
...
PS: Eric Clapton´s Layla works even better, because it was recorded on a Fender Champ.
Gobs of single ended Tubyness.

He recorded it with a tweed.
Sadly, mine is a lowly silver face. :(

PS.: yet another can of snakes: tone caps, carbon composition resistors, hemp cone, etc... You know what I mean if you're into the musical instrument stuff.
 
Last edited:
Carbon resistors have their place. Of all the modern production resistors, they're the least like an ideal resistor, their resistance generally rises with frequency. This flaw can be a virtue - in a RC circuit it adds an increasing cutoff as frequency increases. This can be quite useful for dampening high frequency interference, radio pick-up, and other RFI effects. Plus, it's just cheaper than an inductor or ferrite bead plus resistor, and manufacturers are concerned about pennies that add up.

Hemp cones? Every cone material (and shape) is different (stiffness, weight, internal dampening) and will affect the sound. How and how much is debatable. Instruments can be finicky things and are a sum of all the parts, plus usually for instruments like guitar, distortion is preferred rather than avoided. And musicians are a VERY subjective breed, often prone to emotionalizing rather than rationalizing (I'm the engineer in a family of musicians).
 
Well, in this particular case there is solid Physics justification, besides any psycho-logical or psycho-enhancing overtones :) on its sounding different.
Big way, it´s nothing subtle.

Hemp fiber is *strong* and hemp fiber based ropes and cords are still much in use, 6 times as strong as cotton fiber, let alone wood fiber which is smashed, boiled and blanched while being turned into a paste, so any paper incorporating it will be way stronger, more rigid and as a side note, better self damped, so cone response will vary a lot.

Personally I don´t much like hemp cones for guitar (except, maybe Jazz Guitar) because to my ears it´s darker sounding.

But there measurements match perceptions :D
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.