New 5F6A Build Issues

Fender Amp Discussion

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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I have a fresh set of TAD 6L6GC's which are rated around 30W, as well as a GZ34 rectifier coming tomorrow. I think what I may do, in the meantime, is install a bias pot, to simplify things.
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Re: New 5F6A Build Issues

Post by pompeiisneaks »

Yeah it's a good idea, the Hoffman version comes with adjustable put and I loved it. Traded the amp though.

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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

So, I've installed a 25k linear bias pot, and replaced the 56k bias resistor. (First with a 27k, but it biased too cool, so I twisted two 10k's together, and that got me down into acceptable range, right around 41.5 mA.

I'm going to spend a little bit of time tonight triple checking my connections and solder joints, and then, what's the next step ?
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

What are the power supply node voltages now (measure to ground from each filter cap)? Do you have 6L6GC's and a GZ34 installed now? I would check for voltage on all the preamp tube pins 1 and 6 without tubes, and if they look reasonable (something close to the PI filter's voltage) you can put all the preamp tubes in and measure all pins 1 and 3, and 6 and 8. Plugging in, you should get sound, unless there is some other issue lurking.
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Re: New 5F6A Build Issues

Post by Stevem »

Your lucky pin 6 is not used on those tubes!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

With 6L6WGC's and GZ34 installed, I'm getting 430 to 432v DC on the filter caps, and 426v DC on V1, pins 1 & 6, 422, and 426 on pins 1 & 6, V2, and 423 and 421 on pins 1 & 6, V3.

Upon firing it up after installing preamp tubes, it made a noise that made my wife exclaim (in faux British royalty accent) "Oh Darling, is that the Queen Mary ? She must have made record time !"

It did sound a bit like a ship's horn.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

Power stage plate voltage looks good. Perhaps your primary leads are reversed, and your NFB is PFB?
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I reversed the (blue and brown) primaries, and it did eliminate the foghorn. Now I read -43 on pin 5 of each of the power tubes, but nothing on the resistor to ground off pin 8.
Also, all of the tone pots make a "whoosh" sound, like a presence control when rotated, this doesn't seem normal either.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

So no current flowing at the power tube cathodes, yet you have some sound... that's odd. What voltage do you have on pins 3 and 4? Can you post some photos?
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

Image

Image

Okay, now I'm getting better readings. Pins 3 and 4 are 412.8vDC and 411.6vDC on V4, and 411.0 vDC and 410.8 vDC on V5.

V4 pin 8 = 37.7mvDC, and pin5 = -43.93vDC
V5 pin 8 = 43.1mvDC, and pin 5 = -43.89mvDC

On the preamp tubes, I'm getting the following:

V1- Pin1=160.8vDC, Pin 3= 1.0vDC, Pin 6= 161.6vDC, and pin 8 = 2.40vDC

V2- Pin 1= 177.8 Pin 3 = 1.0 Pin 6= 305. Pin 8= 179.

V3 - Pin 1= 240. Pin 3= 39.78 Pin 6= 240. Pin 8= 39.78
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

Voltages all look good except V1 pin 3 seems low. It should more or less match pin 8, like pin 1 matches pin 6. Could that be a measurement error? Do you get sound?
Last edited by martin manning on Sat Jul 22, 2017 10:55 pm, edited 2 times in total.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I get sound, but there is a bit of a crackle. Actually, apart from the crackle, it's pretty stellar sounding.
Let me re-measure.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

Note the 2.4 V is more like the expected cathode voltage, and since they are tied together they should be identical. Crackling could be bad solder joints, or poor contact between tube pins and sockets. Poke around with a chopstick to look for sensitive areas. Note with 180 V on the cathode of V2, the heater-cathode insulation is highly stressed. That tube should have 200 V spec for Vh-k.
Last edited by martin manning on Sat Jul 22, 2017 11:00 pm, edited 1 time in total.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

They are, indeed, identical. I must have written it down wrong. I'm going to do a little chopsticking, and see if I can't chase down that crackle, but other that, it really sounds fantastic.
Thanks so much for the sage advice and direction.

If I get brave, I may post a clip or two.
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

You're welcome, glad you've got it working. I'm wondering why you lost voltage on the power stage cathodes at one point. Definitely poke around for an explanation.
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