I just finished my first scratch built amp.

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arwokc
Junior tube assistant
Posts: 1
(3/7/04 12:07 am)
  I just finished my first scratch built amp.
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 Well, I may have bitten off more than I could chew here. I wanted a Fender Vibroverb, but didn't want to pay 2 grand for a 40 year old amp and the issues that come with them. So, I figured what the heck. I have the diagrams and the layout of the thing. I'll build one from scratch. First off let me say I didn't use the Hoffman type kit. I built it to look original right down to the cloth covered wiring. I know! I know! No need to go there. What's done is done! I really enjoyed the whole experience. Right up until I turned the power on that is. I pretty well expected to have a lot of hum and cracks and pops associated with my inexperience. But the thing is really quiet. Actually too quiet! If I turn the volume all the way up, I can hear the guitar. It's about the equivalent of the volume knob set on "1" on any other amp with a 50 watt output at 8 ohms. The biasing went really well though. I have one reading 36.9mA and the other reading 36.1mA. If what I've read is right, this is actually very close. I now must start the tedious process of finding my problem using the Hoffman troubleshooting method. In the meantime, if anyone has any suggestions, feel free to throw them my way.

Andy

Edited by: arwokc at: 3/17/04 11:17 pm
 
jbrew73
Junior tube assistant
Posts: 104
(3/7/04 12:22 am)
  Re: I just finished my first scratch built amp.
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 check all the resistor values for mistakes like a 470 in place of 470k. also post all the pin voltages for the pre and power tubes.
 
arwokc
Posts: 2
(3/7/04 1:06 pm)
  tube values
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 OK,
I spent the better part of last night and this morning rechecking my wiring and resistor values. I think I see where you are going with your suggestion. The simple fact that the amp is working it just doesn't have near enough output would suggest too much resistance somewhere. The only value that seemed wrong was the feedback resistor. On the board it was only checking out at about 49 ohms. But when I removed it from the board it checked correctly at 819 ohms for a 820 ohm resistor. So, I put it back in. I just finished checking the pin voltages of the power and preamp tubes. The voltages for both power tubes were 1=-44, 2=8, 3=438, 4=438, 5=-44, 6=438, 7=8, 8=0 gnd.

The preamp tube voltages weren't exactly the same, so I'll list them in a 1=v1/v2 format. 1=243/241, 2=0/0, 3=2/2, 4=8/8, 5=8/8, 6=245/251, 7=0/0, 8=2/2, 9=8/8.

I have tried the amp with the guitar in both channels. These values happened to be with it plugged into the vibrato channel. One odd thing I noticed is that if I turn the reverb knob to its lowest setting, I lose all volume on the vibrato channel. I thought I must have it wired wrong. But after rechecking against the schematic, all appears correct. Hmmmm!
 
6G6
Junior tube assistant
Posts: 114
(3/7/04 2:28 pm)
  Re: tube values
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 OK, what's up with the 8 on the heaters?
Is that 8VDC above ground with the 6.3VAC on top to reduce hum?
 
arwokc
Posts: 3
(3/7/04 4:31 pm)
  heater voltage
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 you got me buddy! I'm using the 022798 PT that I got from Doug. It specs out that the green leads should be 6.3v but there is 8.1v at every tube heater lead connection from the pilot light clear down to the the last tube in the chain which is the normal stage preamp tube.


 
Wanabejimi
Junior tube assistant
Posts: 55
(3/7/04 7:58 pm)
  hmmmm
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 Check te grounding on your output jack. If you have them handy, take two alligator clips wired together and hook one to the chassis and the other to the ground on the output jack. Might try the same on the input jack as well. Good luck
 
arwokc
Posts: 4
(3/7/04 11:10 pm)
  speaker grounding
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 Hey Wannabe,
Thanks for your imput. I thought of that too. I originally ground them and the OPT in accordance with Doug's grounding schematic. I actually even checked the OPT ground as well. I went as far as to mount both the speaker grounds with ring terminals as well as the OPT ground underneath the bolt mounting the OPT transformer to the chassis. I then checked resistence to ground from each jack point to a point on the opposite side of the chassis. There was great continuity through that ground point. There is one point that I am questioning and that is the way the electrolytic capacitors are mounted to the fiber circuit board on the back side of the chassis. I may have misinterpreted the schematic and wired the two 80uf 450vdc Sprague Atoms through the two 220k - 1 watt resistors incorrectly. I haven't found any pictures of the back side of the fiber circuit board to check it against though. I tried an alternate method wiring but that only let to it blowing the fuse as soon as the amp was taken off stand by. I guess I'll have to sleep on it.
 
6G6
Junior tube assistant
Posts: 117
(3/8/04 10:22 am)
  Re: speaker grounding
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 OK, back to the heaters.
If you are reading 8V from each heater connection to ground, then that would be 16V from pin to pin! Too much.
That is AC, right?
From each heater pin to ground should read about 3.1-3.2VAC, 6.3 between pins. It could be off by .2-.3 either way and still be OK, but not by that much.
Did you ever check the (usually green) wires right from the transformer before connecting anything? If you don't have 6.3 volts there, then the transformer's wound wrong.
While that isn't likely, nothing is impossible.
 
arwokc
Posts: 5
(3/8/04 1:08 pm)
  pin to pin
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 I'll double check the voltage from pin to pin when I get home from work tonight. I was measuring from each pin to chassis ground. I also found a schematic for the 220k resistor wiring to the capacitors. I'm going to rewire to that schematic. What I found odd was that the schematic shows voltage readings at certain points and states that it should be within 10%. Prior to me rewiring the capacitors, all my voltages were right where they should be. That should have been my indication that the capacitors were wired correctly in the first place. Gotta stop and get more fuses on the way home from work tonight. Thanks for your ideas 6G6. Keep 'em coming. I have too much money in this thing to give up now.
 
arwokc
Posts: 6
(3/9/04 1:06 am)
  Making progress
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  Okay, tonight was more productive. I wired the two 80uf Sprague Atom caps in accordance with the series wiring diagram I found on Doug's site. This eliminated blowing fuses. Gee, imagine that. After measuring pin to pin voltage on the tube heater circuits, I have 6.28 volts at all tube sockets. That problem seems to have an error on my behalf. Once again, big surprise. According to the schematic, all the voltages are within the limits. The only problem I have now is that I not only don't have any amplification of the guitar, but my speaker emits a horn type noise the instant you take the amp off of stand by. I'm still puzzled about the 820 ohm resistor that measures 46-47 ohms on the circuit board, but reads 819 when removed from the board. I've tried three different resistors and all three do this. This is the resistor in the feedback line from the output transformer. Somehow I think the fact that its inline of the OPT is the reason is reads differently when installed in the circuit. Other than this, all my resistors are checking out, I've double checked all my wires and there connections. They all seem to be sound. One other thing, I can power up the amp without the power tubes installed and I don't get the horn sound from the speaker. It almost sounds like a feedback problem, but I don't know from what. It will make this noise with or without a guitar plugged in the moment I take the amp off stand by. The quest continues. Any ideas or suggestions, don't hesitate to send them my way.

Thanks,

Andy
 
6G6
Junior tube assistant
Posts: 118
(3/9/04 10:39 am)
  Re: Making progress
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 I'm not sure which version of the Vibroverb you're building.
I looked at the AA 763 version and found that it has a 47 ohm resistor going to ground off of the left end of the 820 ohm in the feedback circuit.
If you measured to ground from the PI side of the 820 you would get 47. If the other lead is on the transformer side of the 820, it goes to ground through the transformer and completes the circuit back through...that same 47 ohm resistor.
It sounds like you are getting close.
You might (or might not) be able to stop the noise with 1,500 ohm resistors on the grids of the 6L6s. They stop oscillations and won't hurt anything.
As to the no sound yet part, I think there is still one more deamon hiding out in there.
If it makes noise when it's off of standby, then you have output (although it's not the kind you wanted).
Check upsteam in the preamp and PI sections to see why you aren't getting signal to the power section.
 
arwokc
Posts: 7
(3/9/04 1:10 pm)
  grid resistors
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