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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Aug 23, 2016 3:31 pm
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Thanks Arjay, at least I got the pots replaced and the O/T re-wired today.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Aug 24, 2016 2:25 am
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FWIW... I get my "boutique" or vintage-style parts from Parts Connexion. Stephanie Shcherban is the parts manager. She is nice to work with. Competent.

http://www.partsconnexion.com/capacitor ... _main.html


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Aug 24, 2016 11:54 am
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Thanks, the email must have worked; the order is on the way now. Be here Friday, still kind of a disappointing order turn time and shipping is overpriced. I would not buy from them again, may be a one off thing but there's no impression like a first impression.
Quote:
Parts Connexion
they were not listed on the Jupiter dealer website! But after ordering and some further searching they are one of the few that have Jupiter caps in stock.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sat Aug 27, 2016 10:42 pm
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Well the caps came in Friday about 730pm, I put off the install until Sat. late morning.
Had a few things to sort out and ran a few errands, watched the fights...and then put electrons through the chassis at about 1130pm. Started w/ just the original 5y3 to see what was what, B+ was over 400. Swapped to one of the new 5y3wgta, B+ @ 385, 2nd stage @313, 3rd stage @ 291. The drops seemed relative to the schematics so I shut down and installed all the new tubes and got the following numbers at the plates:
V1-87v
V2-165v
V3-380v
V4-380v
Shut down after that and installed the dual bias tester, both 6v6's measured 38ma @ idle, seems a little hot to me. Shut down after a quick low volume/output check w/ a tele in all the inputs seems fine and the output actually sounded fine through my c12k(100w) test speaker and I didn't adjust the tone control.

So, I have a working amp but I'm a little concerned that I'll burn up 6v6's weekly!!! What do I need to look at to get this dialed in? I quickly took VDC readings @ all the board grommets from the layout. From R-L if looking at the chassis installed in the cabinet. This is what I got(I'm just gonna number them 1-11):
1) -.45vdc
2) -.45vdc
3) 87.5vdc
4) 87.6vdc
5) 2.3vdc
6) 163vdc
7) 165vdc
8 ) 19vdc
9) 291vdc
10) 313vdc
11) 385vdc

Thanks...


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 12:19 am
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You prolly have strong 6V6GT tubes. Cathode biased amps will pull more current wrt how strong the tubes are. As long as the tubes are not red plating or going into OD too soon in the volume range, you should be ok.

Healthy Class "A" ~ 80% or more max dissipation.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 2:17 am
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BMW2002Ti wrote:
You prolly have strong 6V6GT tubes. Cathode biased amps will pull more current wrt how strong the tubes are. As long as the tubes are not red plating or going into OD too soon in the volume range, you should be ok.

Healthy Class "A" ~ 80% or more max dissipation.


+1

I don't see an issue here. Your 380 VDC plate voltage is fine seeing as how an AB763 Deluxe Reverb drives the 6V6GTA's of its output stage with over 400 VDC. And cathode-biased amps routinely run a skosh hotter than the traditional fixed-bias class AB platform.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 7:48 am
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Thanks again for all the insight and advice.

I swapped the V1 w/ the other 6sc7 and got 88vdc, just to check it out.
I retested the 6v6gt w/ the actual plate voltage from pin 3 to pin 8(was measuring incorrectly earlier-only pin 3 plate output to transformer), is 363vdc on both tubes. Also re-measured both sides of V1 & V2, they are both closely matched.

My biggest question still is the -.045vdc reading to ground for V1, the schematic shows -9vdc. What is influencing this reading to be so far from the schematic, resistor values or the actual ground? As far as the layout, it shows a chassis ground @ the two 5meg resistors(they measure 6.4M & 4.8M). My chassis doesn't have a ground solder there, it is grounded at the next spot shown in the layout. Should I be concerned enough to add a new solder point ground?

Also I just realized that I accidentally have the coupling caps reversed as far as the outer foil is concerned compared to the picture in the guitar amp handbook, how much of a difference does it make?

Gave the amp a 30 minute idle test run prior to all the new readings.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 9:36 am
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sfceric64 wrote:
Also I just realized that I accidentally have the coupling caps reversed as far as the outer foil is concerned compared to the picture in the guitar amp handbook, how much of a difference does it make?


Probably none. The polarity of coupling or isolation caps is much less critical than for the electrolytics of a filter array or bias-supply circuit.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 11:55 am
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sfceric64 wrote:
My biggest question still is the -.045vdc reading to ground for V1, the schematic shows -9vdc. What is influencing this reading to be so far from the schematic, resistor values or the actual ground? As far as the layout, it shows a chassis ground @ the two 5meg resistors(they measure 6.4M & 4.8M). My chassis doesn't have a ground solder there, it is grounded at the next spot shown in the layout. Should I be concerned enough to add a new solder point ground?

Also I just realized that I accidentally have the coupling caps reversed as far as the outer foil is concerned compared to the picture in the guitar amp handbook, how much of a difference does it make?



With amp off... measure the resistance between grounding point(s) in question and the chassis. If static DC resistance is above a couple ohms --- look to improve grounding. If not, don't worry. You want to avoid ground loops.

The reason for low voltage (cathode to ground) --- could be a weak tube. You have another you can try? What is the anode voltage, for the triode half in question?

As for the "polarity" of non-polar coupling caps --- as Arjay pointed out... prolly no issue. I simply line up coupling caps so they look consistent on the boards.


Image

Image


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 2:44 pm
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BMW2002Ti wrote:
I simply line up coupling caps so they look consistent on the boards.


+1

The hallmark of a conscientious, professional technician.

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 10:46 pm
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Quote:
With amp off... measure the resistance between grounding point(s) in question and the chassis. If static DC resistance is above a couple ohms --- look to improve grounding. If not, don't worry. You want to avoid ground loops.

The reason for low voltage (cathode to ground) --- could be a weak tube. You have another you can try? What is the anode voltage, for the triode half in question?
That's what I needed, will check it out Monday. So far I have only tested for continuity of grounds and not resistance variance. I tried both of the 6sc7's in V1 and went with the closer to the 92V, it may still be weak though but both sides test very close to each other.

Here's a pic of the chassis side after testing, getting ready to go back together. Then I needed a serious nap, up til 3am then up at 6am for the F1 race!! A little blowout on the 3rd can from a lazy iron hand, epoxied over and will keep an eye on it.
Image

Got it back together after a good deep couch sleep this afternoon and re-claimed my kitchen table.
Played for an hour after supper, really good clean tones from on to about 6. I was taking it easy as the speaker was untested thus far. The amp responds really well to guitar controls and pick attack. The tone on/off pot induces some undesirable distorted signal in the upper range 8-12, may have to look at the caps there but don't know whether it is the pot or the caps. It does disappear if I back off the volume some(may just need to settle in to the new parts/pieces). Also noticed that the mic volume will distort the speaker when plugged into the instrument jack, but induces some hum also in the upper range. Finally the speaker doesn't seem able to handle the bass well when the volume gets to 7-8 and farts out. I didn't push the speaker much past that, but seems overall to be slightly overpowered by the amp. Otherwise the amp is completely quiet up til about 8, then a slight hum.


Last edited by sfceric64 on Mon Jul 10, 2017 10:36 pm, edited 1 time in total.

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 11:04 pm
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sfceric64 wrote:
I didn't push the speaker much past that, but seems overall to be slightly overpowered by the amp.


Unusual, given that the C12K is purported to handle a 100-watt input signal. Possible defective speaker......?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 11:19 pm
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Sorry Arjay, shoulda mentioned I put the p12r speaker back in the cab. I only used the c12k for the initial startup load. Although it is fine up to about halfway on the chicken head, anything much further and its all mids/highs no bottom left in it with a heavy hand. Light picking is ok but still a little flabby.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Aug 28, 2016 11:36 pm
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The P12R is only good for about 20 watts -- it's likely that the amp is pushing the upper edge of the speaker's envelope. Try the C12K again and see how that sounds.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Mon Aug 29, 2016 2:44 am
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sfceric64 wrote:
That's what I needed, will check it out Monday. So far I have only tested for continuity of grounds and not resistance variance. I tried both of the 6sc7's in V1 and went with the closer to the 92V, it may still be weak though but both sides test very close to each other.

Here's a pic of the chassis from each side after testing, getting ready to go back together. Then I needed a serious nap, up til 3am then up at 6am for the F1 race!! A little blowout on the 3rd can from a lazy iron hand, epoxied over and will keep an eye on it.

Image



92 VDC plate voltage seems a bit low. Think that the original schematic had around 150VDC on each plate. Low plate voltage = more idle bias current in those 6SC7. Which maybe contributing to the farting tone, at volume. Try another 5Y3GT rectifier.

Very nice layout! :) One suggestion... physically move the cathode-tied resistor (off the 6V6GT tubes) AWAY from the bypass cap. That resistor gets hot and can heat that cap. Heat can change the cap's electrical behavior and thus change the idle bias of your 6V6GTs. Here's how I did it, in one of my Reverb Unit (which is driven by 6K6GT cathode-bias amp.

Image


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