Picked up a 20t about a year ago, it’s in a different state at the moment but I remember there being some hum in it, would this mod also work for the 20t?
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Ampeg PF-50T Humbuster
- Thread starter mnats
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- ampeg ampeg pf-50t pf-50t
I don´t know. I suppose the topology would work (i.e. inserting a resistor and a cap at the back of the diode) , the values certainly would not. The 20T is 6V6 and more than likely involves a different bias voltage.
I have no 20T available or the schemo, so I do not know which values would be necessary.
I have no 20T available or the schemo, so I do not know which values would be necessary.
That looks really nice! So I have read a capacitor input filter results in higher DC voltage. Is this why the circuit needs the extra 3.3k resistor...to knock the voltage back down to the normal level? Do you know how much the voltage would increase if the 3.3k was omitted?
Doner Designs
Steve Doner
- Jun 2, 2012
- 5,784
- 6,819
- Disclosures
- Doner Designs is an alias for Steve Doner
I didn't have the parts on hand so I ordered them today. The wattage for the resistor was not specified so I assume it's not critical. I ordered both 1/2 watt and 1/4 watt. Would one or the other be preferable?
> Is this why the circuit needs the extra 3.3k resistor...to knock the voltage back down to the normal level?
Yes. As for the value at the back of diode, I can not remember. What I did was to remove the 6L6 and note the range of bias. Then, using Mnats trace and idea of an additional cap, used excel to calculate what res. would work with most of the original circuit. It was about 3,3k, so that is what I choose.
> I ordered both 1/2 watt and 1/4 watt. Would one or the other be preferable?
Does not matter. I would take 1/2: It can not hurt (I think)....
Yes. As for the value at the back of diode, I can not remember. What I did was to remove the 6L6 and note the range of bias. Then, using Mnats trace and idea of an additional cap, used excel to calculate what res. would work with most of the original circuit. It was about 3,3k, so that is what I choose.
> I ordered both 1/2 watt and 1/4 watt. Would one or the other be preferable?
Does not matter. I would take 1/2: It can not hurt (I think)....
Doner Designs
Steve Doner
- Jun 2, 2012
- 5,784
- 6,819
- Disclosures
- Doner Designs is an alias for Steve Doner
Thanks everyone for the advice via this thread and the various links. I got the mod done today with supervision and assist from my son who is an engineer. It works as advertised. There is now zero audible 60 hz hum and no audible degradation of the tone when playing. Also am now able to adjust the bias properly whereas I had previously set it in a position that minimized the hum. Posting pics here for anyone who had trouble seeing the detail in the bertr photo. Got the parts from Mouser.
By the way, I found the secret sauce that makes this amp sound so good - it contains a flux capacitor. Really - upper left in the first full size picture. No idea what that is doing there. Maybe the name of the company that makes the boards for Loud Tech.
By the way, I found the secret sauce that makes this amp sound so good - it contains a flux capacitor. Really - upper left in the first full size picture. No idea what that is doing there. Maybe the name of the company that makes the boards for Loud Tech.
Last edited:
Doner Designs
Steve Doner
- Jun 2, 2012
- 5,784
- 6,819
- Disclosures
- Doner Designs is an alias for Steve Doner
Doner Designs
Steve Doner
- Jun 2, 2012
- 5,784
- 6,819
- Disclosures
- Doner Designs is an alias for Steve Doner
I had a bit of a scare after doing my mod. All of a sudden I started hearing light crackle/static type sounds intermittently. A had a GK amp that did this and GK never could figure out the source so they sent me a new one. To narrow it down I listened with headphones from the two XLR outputs and determined that the source of the noise was the preamp or something before the preamp. Fortunately I had a couple of very quiet J&J preamp tubes that I bought when trying to narrow down the problem with my GK amp. They didn't resolve the GK issue, but did silence the Ampeg. I'm guessing the noise may have been there for some time but I just started hearing it after eliminating the hum which probably was masking the intermittent light static. Now she's dead silent. Whew!
You guys are getting me to want to do this mod. I would have to pay to get it done, but am considering it.
Maybe you can shame Ampeg into doing it for you?
Doner Designs
Steve Doner
- Jun 2, 2012
- 5,784
- 6,819
- Disclosures
- Doner Designs is an alias for Steve Doner
You really should Jimmy and then report back to Ampeg. I’d be happy to send you the parts.
Maybe you can shame Ampeg into doing it for you?
Neither one of those scenarios can happen for different reasons. The Ampeg reason is obvious. The Steve reason can't happen because I can't solder worth a crap and never will. But I do thank you very kindly for the offer, Steve!
Not to me - would anyone care to enlighten me? If you endorse a company's products you're not allowed to ask them to fix an obvious design defect even if you can supply them the fix
?
I could show it to them, but I'm not about to ask them to do it for me. I'm no schnorrer. Chances are they've already seen it anyway.Not to me - would anyone care to enlighten me? If you endorse a company's products you're not allowed to ask them to fix an obvious design defect even if you can supply them the fix
Oy vey LOL. Dunno if there's a Yiddish translation but one of my favs is "If you don't ask, you don't get"
. I'd think it would be to Ampeg's advantage to listen to their endorser's needs but I guess those days are gone 
.OTOH I have to head down to the post office and pick up a vintage MOSFET-based B15 clone I scored for $100
. 100 watts so I could decapitate it and make it into a V4B clone 
.
Trust me...they listen to me. But I'm not about to ask anyone there to do this mod for me.Oy vey LOL. Dunno if there's a Yiddish translation but one of my favs is "If you don't ask, you don't get"
You guys really have to stop turning off-handed comments into evil corporate plots.
Well, they don't let us talk about poly-ticks so all's we got is evil corporate plots and buying gear we don't need to keep us from wasting our time practicing LOL.
After spending more time with my pf-50t, I think I'm going to order the resistor and capacitor soon. the rebate from buying the head and 115 cab should just cover the costs of the parts and the fee of the tech I'm going to have do the mod.
