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PostPosted: Fri May 01, 2015 10:00 am 
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Ok, I finished my Tweed build. This is my second Trinity build (built the 18 watt TMB first which has become my main amp ever since I finished building it!). My confidence level and ability with the second build was much higher than the first and I was very diligent in checking things as I went along (checking continuity, resistor values, etc.). Everything seems to have come out really good with one exception which I will lay out below with my voltage measurements. The amp works, has sound, etc, but I think I am not getting the natural overdrive I am supposed to get with this amp (although I haven't given it a full test yet since I was still checking voltages, etc.).

So, here are my voltage measurements (the second number in parentheses represents the number per the documents that came with the kit - this is the number with JJ tubes). All measurements are DC voltages unless indicated with VAC.

First set of measurements with no tubes installed:

Outlet I am using: 123 VAC
V5 pins 4 & 6: 370 VAC (supposed to be greater than 350 VAC)
V5 pins 2 & 8: 5.7 VAC (5.0 VAC)
V4 & V3 pins 2 & 7: 6.5 VAC (6.3 VAC)
V2 & V1 pins 4/5 & 9: 6.5 VAC (6.3 VAC)
Pilot light (between the two connection points): 6.5 VAC (6.3 VAC)

Second set of measurements with V5 installed, standby switch 'on' (as-if sound would come out of a speaker):

V4 & V3 pin 3: 505 (supposed to be greater than 400)

Third set of measurements with all tubes installed and standby switch 'on':

V5 pin 8 (B+): 383 (370??) I am unsure about what this one is supposed to be as I have had several different measurements depending on if tubes are installed, standby is 'on' or 'off', etc.

V1 pin 1: 116 (120)
V1 pin 3: 1.88 (1.8 )
V1 pin 6: 118 (120)
V1 pin 8: 1.88 (1.8 )

V2 pin 1: 144.5 (148)
V2 pin 3: 1.24 (1.3)
V2 pin 6: 186.5 (191)
V2 pin 7: 2.5 (17)
V2 pin 8: 45 (45)

V3 pin 3: 376 (356)
V3 pin 4: 332 (326)
V3 pin 8: 21.2 (20)

V4 pin 3: 376 (356)
V4 pin 4: 332 (326)
V4 pin 8: 21.2 (20)

The one glaring variance is V2 pin 7 which measures around 2.5 VDC (supposed to be 17). I checked continuity all around this area and also reapplied solder on the board at the 1M resistor (R12 I believe) at the eyelet closest to the side of the board. The measurement before resoldering was 2.57 but was 2.45 after resolder. Continuity checks out (before and after the one resolder point) and the color codes on the resistor I am using indicate that it would be a 1M resistor. This is the one resistor, however, that I don't think my multimeter can check (my meter has the following options: 2000k, 200k, 20k, 2000, 200). Could it be the tube?

I also wonder about my values on V3 and V4, pin 3. Are mine too low or ok?

All my other measurements look good to me but hope to hear from more experienced builders.

One other issue I have is that the standby switch seems to be wired opposite of what I would want - i.e. when the switch is toward the "standby" wording on the chassis, then I get sound from the speaker. I would expect no sound when in the "standby" position. Is this just a matter of swapping the leads? I am assuming yes but wanted to ask.

Thanks in advance - hopefully there is a straight forward solution.


Last edited by Bigfoot on Sat May 02, 2015 7:54 am, edited 1 time in total.

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PostPosted: Fri May 01, 2015 1:26 pm 
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Your voltages look good, all in check. How does the amp sound?

Did you install the optional grid stopper smoothing resistor?

Sometimes grid voltages are hard to measure because the node is so high impedance it loads down your volt meter and causes an inaccurate reading.

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PostPosted: Fri May 01, 2015 1:56 pm 
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I definitely need to spend more time playing through it but I can say that it has a nice clear as a bell clean sound - I have not had opportunity to turn up the volume since I have been working on it in the middle of the nights and the rest of the family is sleeping on the floor above me ;)

Is there no concern or unwanted result with the V2 pin 7 measurement being so low? Also, to answer your question, I did not install the optional smoothing resistor - should I? What is its function? Also, I don't understand the instuctions where they say "make sure the resistor goes right on the pin as it is acting as a 'grid=stopper'. does it go directly in-line with the flying lead coming into pin 7, with the other side of the resistor attaching directly to pin 7?

thanks for your response.


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PostPosted: Fri May 01, 2015 4:26 pm 
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Bigfoot wrote:
I definitely need to spend more time playing through it but I can say that it has a nice clear as a bell clean sound - I have not had opportunity to turn up the volume since I have been working on it in the middle of the nights and the rest of the family is sleeping on the floor above me ;)

Is there no concern or unwanted result with the V2 pin 7 measurement being so low? Also, to answer your question, I did not install the optional smoothing resistor - should I? What is its function? Also, I don't understand the instuctions where they say "make sure the resistor goes right on the pin as it is acting as a 'grid=stopper'. does it go directly in-line with the flying lead coming into pin 7, with the other side of the resistor attaching directly to pin 7?

thanks for your response.

It smooths the amp's tone out at high volume/distortion levels. It goes right on (in series) with the wire going to pin 7.

I asked because it could slightly change the voltages.

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PostPosted: Fri May 01, 2015 10:47 pm 
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Looks like your amp is working properly. Crank it up and see how it sounds!

Standby switch terminology is not logical. "Closed" and "open" is more appropriate than "on" and "off" when referring to it. The amp is in standby when the switch is open, but most of the time a switch is considered "on" when it's closed.

If you have a single pole single throw standby switch (two lugs), the only way to reverse the operation is to physically rotate the switch 180 degrees. Swapping the wires will have no effect.


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PostPosted: Mon May 04, 2015 6:24 pm 
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What has been said already. It is hard to get a good reading on that grid pin. We didnt use to even include it, so you should be good to go. What rectifer you use , and your mains voltage will determine your B+ . The JJ is close to NOS values.

Tubes are not identical so there is plenty of difference in measurments. Our are provided as guidance for you.
i dont think there is anything wrong with your amp. At about 3 or more, you should start to get bluesy break-up

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PostPosted: Mon May 04, 2015 6:48 pm 
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Thanks everyone.


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