Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ats on June 04, 2017, 07:10:36 am
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Hi all , just picked up a 1954 Valco Comet/ Spectator that matches the 510-11 schematic. I am replacing the parts that have drifted to far out of value and would like to see if i'm getting the expected readings but i can't seem to find anywhere the expected voltages as the schematic doesn't have that info. Any help would be greatly appreciated.
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Just measure the AC voltage feeding the rectifier. Then measure the DC voltage at each filter cap. Finally, measure dc voltages at every tube pin, even if the voltage reads zero. Post all the voltages, preferably directly on your schematic, and we'll tell you if they look reasonable.
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schematic attached is of a clone of the same amp with 2 mods: the dual SE and the added pentode front-end. we built entirely with vintage NOS components except for the OT.
your measurements for plate and screen voltages should be similar, and your preamp values may be higher - this just for reference and should not be taken as exact.
as slucky states - measure the ACV of PT across pins 4 & 6 of the 5Y3 and you should be able to calculate the expected B+ output.
BTW, our amp has been going strong since 2007... :icon_biggrin: a punk rocker own it now. it gets beat down. hard.
--pete
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Just measure the AC voltage feeding the rectifier. Then measure the DC voltage at each filter cap. Finally, measure dc voltages at every tube pin, even if the voltage reads zero. Post all the voltages, preferably directly on your schematic, and we'll tell you if they look reasonable.
Thanks sluckey i will compile that info to post here by the weekend. Been pretty hectic the last couple days.
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schematic attached is of a clone of the same amp with 2 mods: the dual SE and the added pentode front-end. we built entirely with vintage NOS components except for the OT.
your measurements for plate and screen voltages should be similar, and your preamp values may be higher - this just for reference and should not be taken as exact.
as slucky states - measure the ACV of PT across pins 4 & 6 of the 5Y3 and you should be able to calculate the expected B+ output.
BTW, our amp has been going strong since 2007... :icon_biggrin: a punk rocker own it now. it gets beat down. hard.
--pete
Thanks pete . i bet that amp sounds cool. is that your design?
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Hi all , just picked up a 1954 Valco Comet/ Spectator that matches the 510-11 schematic. ... i can't seem to find anywhere the expected voltages as the schematic doesn't have that info. Any help would be greatly appreciated.
It might take me a few days (just like a different request to verify an OT primary impedance), but I have a Supro 510-11. If impatience doesn't get you first, I'll measure the voltages in my amp for you to compare.
Caveat: My amp had already been serviced before I bought it, and drifted components & old filter caps replaced.
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schematic attached is of a clone of the same amp with 2 mods: the dual SE and the added pentode front-end. we built entirely with vintage NOS components except for the OT.
your measurements for plate and screen voltages should be similar, and your preamp values may be higher - this just for reference and should not be taken as exact.
as slucky states - measure the ACV of PT across pins 4 & 6 of the 5Y3 and you should be able to calculate the expected B+ output.
BTW, our amp has been going strong since 2007... :icon_biggrin: a punk rocker own it now. it gets beat down. hard.
--pete
Thanks pete . i bet that amp sounds cool. is that your design?
thanks, amp sounds nice over-driven - as far as clean playing, it's <= 10W.
design was a collaborative effort by a good friend and self.
--pete
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Hi all , just picked up a 1954 Valco Comet/ Spectator that matches the 510-11 schematic. ... i can't seem to find anywhere the expected voltages as the schematic doesn't have that info. Any help would be greatly appreciated.
It might take me a few days (just like a different request to verify an OT primary impedance), but I have a Supro 510-11. If impatience doesn't get you first, I'll measure the voltages in my amp for you to compare.
Caveat: My amp had already been serviced before I bought it, and drifted components & old filter caps replaced.
Thank you , if you have the time that would be cool!.
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I finally got some time to get the reading off the Valco here is what i got. Hope this helps.
5Y3 readings:
(Pin 1) 0
(Pin 2) 268v
(Pin 3) 0
(Pin 4) 281vAC
(Pin 5) 0
(Pin 6) 280vAC
(Pin 7) 0
(Pin 8) 269v
6V6 readings:
(Pin 1) 0
(Pin 2) 0
(Pin 3) 254v
(Pin 4) 205v
(Pin 5) 1v
(Pin 6) 0
(Pin 7) 6.1vAC
(Pin 8) 26v
6SL7 readings:
(Pin 1) loud buzz when probed
(Pin 2) 101v
(Pin 3) 1.2v
(Pin 4) loud buzz when probed
(Pin 5) 1v
(Pin 6) 1.2v
(Pin 7) 6v
(Pin 8) 0
(http://allthesound.com/2017/valco/v6.jpg)
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looking at the drawing: voltage readings for 6SL7 pin 5, & 6 seem dubious. if your B+ measurements are accurate & the 6SL7 is in decent shape, you should be ~100V at pin 5 and ~1V at pin 8.
you stuff mixed up in the list.
the readings in the drawing contradict those in the list. looking at the readings from the drawing, those seem normal. measure DCV at pin 5 & 6 of 6SL7. those 270K resistors also tend to drift up quite a bit - you want to check them too - if they are within 20%, i'd leave them be.
--pete
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looking at the drawing: voltage readings for 6SL7 pin 5, & 6 seem dubious. if your B+ measurements are accurate & the 6SL7 is in decent shape, you should be ~100V at pin 5 and ~1V at pin 8.
you stuff mixed up in the list.
the readings in the drawing contradict those in the list. looking at the readings from the drawing, those seem normal. measure DCV at pin 5 & 6 of 6SL7. those 270K resistors also tend to drift up quite a bit - you want to check them too - if they are within 20%, i'd leave them be.
--pete
Thanks for pointing out my mixup Pete , guess i should have taken a sip of my coffee and woke up a lil before i posted LOL. I just edited the figures. I'm going to fire it up and recheck everything right now , it was pretty late last night when i did it and i may have misread some things , i'll update in a few.
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Just got done rechecking everything. this time i made sure my variac was set to 120v on the button. i think it was 117v last time. here is the updated results. thanks again gents i really do appreciate the help.
(http://allthesound.com/2017/valco/v7.jpg)
(http://allthesound.com/2017/valco/v8.jpg)
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cool old amp. thanks for the pics - and update. the 2200 cathode R connected to 6SL7 pin 3 could probably stand to be swapped - it's not hurting anything, just probably not sounding like it did when new though.
--pete
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if you really want to kick it in the @$$ - tack a 22-47uF 50V electrolytic cap across the 330 ohm cathode R connected to pin 8 of the 6V6. see attached.
--pete
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Brown stripes on tweed fabric...I love that look!
My eye also was drawn to the voltages on the rectifier tube. The two secondary taps are around 282 volts AC each. But the rectifier only puts out 271 volts DC. Why would that be? I am used to seeing the B+ voltage slightly higher than the AC on one leg of the secondary tap.
--Craig
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My eye also was drawn to the voltages on the rectifier tube. The two secondary taps are around 282 volts AC each. But the rectifier only puts out 271 volts DC. Why would that be?
Could be several factors. The 5Y3 will account for a significant voltage drop. That 65 year old filter cap can has probably dried out and needs replacing. Or the PT may be a bit small for the load and also have a fairly high internal resistance.
Filter caps is my number one suspect. I'd replace it even if doing so does not increase the voltage. I bet it will though.
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for a new can cap i'd use this one (https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-10101010-f-electrolytic) and bond 2 x 10uF for the 6V6 plate supply.
--pete
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for a new can cap i'd use this one (https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-10101010-f-electrolytic) and bond 2 x 10uF for the 6V6 plate supply.
That cap is 1-3/8" diameter. It will not fit that 1" can mounting plate. If you get this can be sure to get the matching metal mounting plate and be prepared to make the hole larger.
AES used to sell those 3 section 1" diameter cans. They still sell the mounting plates. I bet they still have access to the 1" three section cans that are a drop in replacement for your old can.
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Thanks for all the replies guys i'm going to order the multi cap Pete recommended i will post some more updates this weekend when i have a lil time.
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i have this one. it's NOS and will need reforming. it's close enough in value to work and will fit without chopping up the chassis. i tested it at 15V and all 3 sections measure 18uF. you're welcome to it for the cost of postage plus a couple of bucks for a cup of coffee.
--pete
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i have this one. it's NOS and will need reforming. it's close enough in value to work and will fit without chopping up the chassis. i tested it at 15V and all 3 sections measure 18uF. you're welcome to it for the cost of postage plus a couple of bucks for a cup of coffee.
--pete
Thanks for the offer Pete! I ordered some 10uf axial caps to use but if i hadn't i'd take ya up on it. Btw i have quite a few NOS multi can caps if you ever have a need let me know.
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Sorry for not following up sooner guys just finished up a huge job that had me really tied up Now i have a lil breathing room again i can get back to finishing this Valco up.
A little back story on this amp. A while back i did a little spring cleaning and took a trip to the dump and the guy that worked there saw an old amp case i was throwing away and said "thats the second one today, hey ya wanna see something ?" he goes into the bin and pulls out the Valco , it was dripping wet it had been raining for 2 days and it looked as if it had been left out that whole time. the plywood on the bottom of the cab was so wet it started to de-laminate and the paper speaker cone was wet to the touch . I removed the speaker as soon as i got home and placed it face down on some newspaper and let the whole thing sit for a few weeks before i even touched it again. i brought it outside on sunny days a few times to help dry it out.
Here are a few shots the day after i got it.
(http://allthesound.com/2017/valco/v1.jpg)
(http://allthesound.com/2017/valco/v2.jpg)
(http://allthesound.com/2017/valco/v3.jpg)
(http://allthesound.com/2017/valco/v5.jpg)
I got the 10uf caps installed, i snipped off the tabs on the old can cap and soldered the new caps to them and grounded them on the base plate of the old can . it seems to be working really well, sounds great! I'm really amazed the speaker survived the soaking.
(http://allthesound.com/2017/valco/v4b.jpg)
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he goes into the bin and pulls out the Valco
is another mans treasure! Nice find
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Just got done rechecking everything. this time i made sure my variac was set to 120v on the button. i think it was 117v last time. here is the updated results. thanks again gents i really do appreciate the help.
(http://allthesound.com/2017/valco/v7.jpg)
(http://allthesound.com/2017/valco/v8.jpg)
what is the width and the length of that chassis?