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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Jun 05, 2013 9:15 pm
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Don't like the situation, Man!! I can accept $130-150 for good components(depending on the number of parts). But the $430 must be an oversight. The fact that you were not advised after the diagnosis, is absurd and a disrespectful business practice IMO. Don't know what kind of agreement you had with Clark. But that sure isn't the way I conducted repairs with folks......Wasn't aware the rectifier tube failed!! Agree with Bill on all counts, except that the cap off the switch is desiginated to fail open. That cap is not optional!!! One type of design is the only acceptable piece. And that's what Fender used in that position. Art

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Last edited by aclempoppi on Thu Jun 06, 2013 5:51 pm, edited 1 time in total.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 06, 2013 4:11 am
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Chris,

Sorry, I am late to this part of the Clark Amp story.

" Pretty sure my rectifier tube died. " I haven't seen many recitifier tubes work well for 12 years --- then short out. Not, unless there was a failure downstream (like in the PS stage or shorted output tube) which caused an excessive current draw during turn-on --- leading to arcing across the rectifier. Most just get weaker and weaker. With resultant power loss in the amp. Those that short, usually do it soon after being pulled from box. Due to poor QA/QC.

I kinda had a little suspicion about this guy, when he originally told you that a leaky coupling cap off the phase inverter caused the fuse to blow. In worse case, this could fuse a control grid of the 6L6GC and lead to that tube failing. But, the current draw would not be as catastrophic, as say the cathode fusing to the anode. Or a heater line fusing.

Anyhow, FORGET the cap off the standby switch --- unless you go to a healthy +1KV rated good quality cap. I mean US made Mallory or Sprague with "real life" ratings.

I have a feeling that your amp's bias section failed. Allowing a lot of current to pass through those output tubes. Now, if they are good 5881s (like mil spec Soviet made ones or US made TungSols) --- you could pass 5-10 amps through that section no sweat --- before the tubes blow up. Thus frying fuses, wires, resistors, even (UGH) output tranny windings. From your description --- the clue is that micro-multi turn bias pot. This is a part that should NEVER see the power section of a tube amp. Hell, I pull & replace them on good solid-state amps. If you like multi-turn pots --- use metal mil spec ones. No plastic junk.

I have one sugeestion. Get a matched pair of 1/2-watt, 1-ohm, 1% precision Riken or Kiwame (or similar flame-proof) resistors. Replace the grounding strap from each cathode (pin 8) of the 5881 to the chassis with a resistor.

Now, if the tube starts to pass too much current... those resistors will pop. Stopping excessive flow.

You may also want to look into putting "flyback" diodes from the 5881 anodes to ground. Esp if you have a BIG OP tranny and a rather weak standby switch.


Good luck with all that!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 06, 2013 5:24 am
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The whole thing started with.................
I was jamm'n along and the amp started humming, i took off towards the amp and it blew the fuse before I got there, The one 5881 had gotten so hot it scorched the back panel and seamed as if it had died ?

I replaced the tubes and it worked fine again for a while, then there was the smoke from the standby switch etc...............
I also saw the (mullard) GZ34 flash just before it died.
My junk tube tester does not even have a spot for 5AR4 on it so I can't even test it.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 06, 2013 8:21 am
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Chris,

I don't think the initial problems were the 5881 or the GZ34. But, they were prolly victims of some current surge through the amp. Blown standby switch can be a hint. Most of these newly made ones cannot handle 400VDC RMS. And may fail spectacularly, if they "see" high transient voltage or current.

Better go over the PS section caps and resistors. And the bias power supply very carefully, before proceding to any rectifier, switch, or output tube issues.

From what the tech/builder described to you and the cost he charged you --- I'd have some suspicion as to whether he did a proper job of fixing the actual problem.

You'd be surprised many "builders" just copy some known product, put it into a fancy box, and spend money selling and advertising their boutique product. But, have little experience keeping the things running. There really aren't that many Mark Sampsons and Ken Fischers. And most of those guys go out-of-business, due to high $$$ of quality parts and good labor.


Good luck. Keep us posted.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jun 09, 2013 5:43 am
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Lets face it Clark amps have a pretty good reputation for being solid builds and awesome sounding amps.
However he has announced his retirement from amp building several times.
I'm thinking he lost his love for amps.
Not much love for his customers either !

I e-mail asking for tracking and he says cant get around to shipping till next week.
Thats after blowing off two other e-mails I sent earlier this week.
I would say his glory days in the amp bizz are coming to an end.
You pay close to 3K for one of his Piedmonts the way mine was set up, you kinda expect a little more customer service and a bit more communication too.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 13, 2013 7:34 am
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Update on the Clark amp

Beemer was right again !!!!
Should have just sent it to you Steve !!!

Came back doing the same thing !
I put another set of 5881's in the amp The power tube closest to the rectifier went from 31ma to a stable 37ma after fifteen minutes of run time, The next power tube started at 28.5ma and as it warmed up I heard a distinct "pop" and the meter went to zero.
I turned off the amp and retested with the same results.

He wants me to send it back :evil:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 13, 2013 12:21 pm
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OK

Might be time to eat some crow
I put in a known pair of 6L6GC Tung-sol RI's and they held together at 1ma or less apart for over twenty minutes without incident.
Gonna let em stay in there till the 5881's arrive

Looks like that last pair of vintage 5881's also contained a dud ! :roll:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 13, 2013 3:46 pm
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Chris,

Good luck! Keep that bias meter connected throughout the test. As 6L6GC can carry more current than typical 5881/6L6WGBs. Thus more postential damage, if shorted.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jun 13, 2013 9:17 pm
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Chris, what kind of plate voltage were you seeing with the 5881's? Also, were they Tung Sol? Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Jun 14, 2013 10:56 am
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I didn't put a meter on it.
They were all Sylvania 5881's
Mostly old organ pulls so............

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jun 18, 2013 2:52 pm
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Here is the deal with the piedmont

It was my bad tubes and junk tube tester that caused the mess
Mr Clark basically did about three times more than he even told me he had done, making the amp as new again.
I think the only thing he didn't mess with was the trannies.
The amp ran like buttah with the 6L6GC's for about a week, now it has the Tung-Sol RI 5881's in it and it sings like an Angel ! No really it is an amazing sounding amp !!!

The only complaint I have is a lack of communication, telling me everything he was doing to the amp before handing me a bill over 400 bucks would have been the thing to do !

My amp is now completely filled with copper foil PIO caps and all components are up to spec.
I have never heard an amp sound better, Never !

Wanna see something fun ? Check out my 1969 Princeton thread ! its coming along very nicely.

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jun 18, 2013 5:50 pm
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Glad to hear everything worked out, with the Clark!!! Seems like you have a very nice choice of amps. Lots of different sonic choices!!! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jun 18, 2013 5:53 pm
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Chris,

Sounds good! It would have been nice to know what exactly caused those Sylvania 5881's to short out.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jun 18, 2013 5:57 pm
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Hey Art I'll be close by in August
The wife wants me to tag along to Pheasant Run Resort for an expo she attends every year.
I'll be stuck in a room or pool for three days.
like 20th through 22nd or something close to that.

Ya the music room is getting full again !

Steve it sounds like Mr Clark found and fixed stuff and just never mentioned it, My guess is he found lotsa trouble and fixed it all.

Sure wish I had a way to test that Mullard GZ34 that was in there at the time of smoke and noise !!! Lucky for me I still had a backup to put in the Piedmont, The Princeton uses a 5U4GB which I have a bunch of NOS

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jun 18, 2013 7:04 pm
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Yeah, Man!!! If you're travelin' light, I could bring a strat or tele and small amp for you. An acoustic would be cool for me. We'll get you folks kicked out of there in no time!!! No, really that's only 1&1/2 hrs down the road, easy drivin' for me. If you're bringin' the kids, we can forget the guitars, and I'll bring my 8yr old grandson, he's a pool hound!! Art

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