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Old 23rd September 2007, 08:32 PM   #1
Minion is offline Minion  Canada
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Default need a little help from some tube Gurus!!!

Hi folks....i have this old Garnet Gnome Tube amp from the 70"s lying arround for quite a while that has seen better days and I want to try to get it working again.....

The guts of the amp are totally shot as most all of the resistors have disintegrated and many of the caps are leaking pluss all of the tubes are shot and many of the wires are corroded...

The Power transformer and output transformer seem to be fine though as well as the speaker , so I was planning to use the good parts and rebuild the amp, accept I am going to leave out the tremello curcuit and the tone controlls, if I wast tone controlls later I can hard wire them in......

I haven"t really worked with tubes much so i don"t understand many of the aspects of working with tubes but I have done a quite bit of solid state stuff so I"m not totally in the dark were electronics are concerned......

So what I have done so far is I found a hand drawn schematic of my Garnet amp and I have designed a simple PCB (I suck at P2P) and I was wondering if someone could look at it and tell me if the schematic looks OK and if my PCB looks OK???

It is a Simple design useing one 12ax7 and one 6L6 and is maybe 5 or 10w, I removed the tremelo curcuit and I didn"t use the tone controlls and just put a 1m Volume pot in between the 2 Triodes.....

Thanx for any help you can give......


Cheers
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Old 23rd September 2007, 09:11 PM   #2
Minion is offline Minion  Canada
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Old 23rd September 2007, 11:09 PM   #3
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Hi Minion

I think that the layout where the 100k resistor is connected over the heater traces won't do good to the noise performance. But don't reroute them to the other side either, but follow Morgan Jones (building valve amplifiers, available at amazon) recommendation that, when using PCB's, the twisted heater wiring should be soldered as close as possible to the socket pins (or straight to the socket pins).

I haven't looked for the schematic, other people are better at that!

Good luck, Erik
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Old 23rd September 2007, 11:38 PM   #4
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The first thing I noticed is how similar it is to a blackface Fender Champ (schematic here.) The unbypassed 150R resistor in the cathode of the second stage is a telltale sign that the amp originally used some negative feedback from the speaker terminal (see the Champ schematic.) I'm thinking that with an 8 ohm output transformer something like 12k between the speaker and the top of the 120R would work. There's about a 61% chance that you'll get the speaker leads mixed up and the amp will squeal like a 13 year old girl....

You are missing a grid resistor on the 6V6. It's important; don't leave it out.

I'm inclined to think that the 10k/2W resistor in the power supply should be 1k. 10k would work, but I'm suspicious.

As for your pcb, it looks to me like the tubes are much too close together. I'd space them at least an inch apart. Two inches or more would be better. Those electrolytic caps should be even farther from the output tube; they'll get too hot where they are.

Personally, I'd build the thing p2p. It's a simple circuit and most of the components can hang off the sockets, pots, and jacks. It would make it easier (possible) to change components later. I feel that pcb's are good for integrated circuits, saving space, and saving money if you planning on making and selling millions of units But I think they absolutely suck for diy tube stuff. That's just my opinion and it's worth what I'm charging you for it.

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Old 24th September 2007, 12:10 AM   #5
Minion is offline Minion  Canada
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Hi, thanx for the tips....

I don"t see a grid resistor in either the Champ schematic you posted or in the schematic I posted (pin 4 and 5 of 6v6)...

can you please clarify??

Thanx
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Old 24th September 2007, 09:51 AM   #6
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The Champ has a 220k resistor from pin 5 to ground. It is absolutely essential.
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Old 24th September 2007, 12:13 PM   #7
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Yers the grid resistor is essential (as is the dc return on an op amp)
Also you seem, on the schematic, to have shorted the secondary of your transformer.
Your main rectifier seems to be two diodes in series (hardly required). If you wanna use two then put 250K across each to share the back-voltage With diodews costing pennies a full wave bridge would be better.
It is conventional to draw the + HT line ABOVE the valve circuit, and the chassis or ground line below it.
Good luck
Tubes/valves are much more forgiving that solid state. just turn off the power if the anodes glow red hot
John
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Old 24th September 2007, 07:17 PM   #8
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You don't have a return ground path for the B+ (HV). You need either a center-tapped transformer (which is grounded) or a bridge rectifier. (google it) Also don't forget the 12AX7 is a 12V filament unless you use the common "center tap", then each 1/2 is 6.3V. And why two fuses?
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Old 24th September 2007, 10:26 PM   #9
Minion is offline Minion  Canada
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Quote:
Originally posted by mbielman
You don't have a return ground path for the B+ (HV).
How do I fix that?? sorry, I just don"t know much about tubes so i am just going from the schematic....

i have the original Transformer from the amp so I"m sure it has the correct voltages and such, but I will test before I begin....

I don"t know why there are 2 fuses but the amp it"s self has 2 fuses like in the schematic.....


Thanx
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Old 25th September 2007, 06:17 AM   #10
mrwhy is offline mrwhy  United Kingdom
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It probably has two fuses because it is a winding with a center-tap: say 120v 0 120v.
Each 120v would have a fuse - not the o v center tap.
A bridge rectifier (ten cents) would be fed by this center-tapped winding. It gives full wave rectification : much better than your half-wave schematic
To get the ground right do this:
Take away the short you have put across the supply!
Then the negative wire (fron the rectifier) is grounnd (chassis) and the + wire is HT + (B+) to feed the anodes.
John
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