New Fender "type" amp build suggestions

[JMPGuitars]

Slow n steady wins the race.

That's the key!

The one that looked light on solder appeared to be not perfect, but within an acceptable range. Whether it's a good connection or not you should be able to tell better in person...and with tools if necessary.

BTW- I have a magnifying light and a digital microscope. Nothing to be ashamed of.

[foreverstrung]

So today I was chasing down some voltages. If you’ve got a chart that would be helpful. I do have general outline of voltages I’ve found online.
I did have a couple issues. ChatGPT and Grok are worthless. What a waste of an afternoon going back n forth with AI
I use a variac and a dim bulb tester in series. With the tubes out, I get about 425DCV at the plate. No light with the tester.
When I put the power tubes in I get 325v at the plate and the same at pin 4….. -34.5 at pin 5 neg bias. ….pins 1/8 0v and my test light comes on. Not strong, but still a low dim light.
I did find 2 problems/mistakes I made troubleshooting today and repaired them. They were not related to this plate voltage issue
To confirm one point we discussed earlier, the OT CT does connect to this side of the 150R resistor, right? Not directly to A node? I’m looking for verification because of the 420v at the plate with no tubes and then getting 325v with the tubes set. Im attaching the layout with this area.
Thx
Dave

[JMPGuitars]

I’m looking for verification because of the 420v at the plate with no tubes and then getting 325v with the tubes set.

I don't know what the voltages are supposed to be for this amp, but I can tell you- in every amp, unloaded voltage (no tubes) will be higher than loaded voltage (with tubes).

[Bieworm]

That solder terminal at V3 is to put the 220k resistor to ground in the reverb recovery circuit. It is present on most blackface reverb schematics.

I recall you have a higher HT voltage than my build, which has 300-0-300V. That would make my voltage chart meaningless for you. Don't have a filled in version on my laptop, but here is the blank version.

325V plate voltage seems very low to me. I recall my plate voltage at 370V with my lower HT winding voltage, so yours should definitely be higher. Possibly there are some tweaks to be made at the bias circuit resistor values. To be sure I need to open mine up and verify what I've got in there. I recall tweaking in that area. In the same time I will take my voltages. What value is the voltage dropping resistor to the bias circuit on your build? On my builds I landed on 160k. I put some resistors in series to achieve that value.
What voltage do you read at the HT before the rectifier?
Also, the HT fuse on the turret board should be 500mA, not 1A

[foreverstrung]

What value is the voltage dropping resistor to the bias circuit on your build? On my builds I landed on 160k. I put some resistors in series to achieve that value.
What voltage do you read at the HT before the rectifier?
Also, the HT fuse on the turret board should be 500mA, not 1A

That voltage dropping resistor is 160k

Before rectifier diodes I’ve got 539v AC, measuring both leads. 269v one lead to ground.
329 DCV after rectifier diodes

I did use a 1a fuse as it was what I had on hand. I’ll replace asap.
Thx
Dave

[foreverstrung]

Just for clarification. I’ve attached print out on this PT data. Hopefully I’ve got it hooked up correctly.

Primary I’ve got both 120v wires (black) to my power switch with a 2A slow blow fuse in series.
Orange wire to ground

Secondary
Red (2) to rectifier diodes
Red/Yellow to ground
Red/Green (not used)
Yellow (2) (not used)
Green (2) to heaters
Green/Yellow (not used)

Appreciate you guys double checking my work

[JMPGuitars]

Green/Yellow (not used)

Green/Yellow is the heater center tap, and should be connected to the power amp star ground.

[foreverstrung]

Green/Yellow is the heater center tap, and should be connected to the power amp star ground.

Thanks! Glad I ran this by you

Another thing noteworthy, besides the plate voltage being low, I’ve got a tube bias meter with both tubes connect simultaneously to check my bias. This is what’s happening. When I turn the bias trim pot all the way down, I get like 8mA and there is no light at the dim bulb. When I turn the trim pot all the way up, it max’s out at about 20m. Way low of course maxed out and the dim light tester lightly illuminates.
Maybe that’ll tell you something that you’ll share with me.
Thx
Dave

[JMPGuitars]

Are you testing your voltages and bias with the dim bulb tester in place?

Follow-up questions: What is the purpose of the dim bulb tester, and how does it work?

[foreverstrung]

Are you testing your voltages and bias with the dim bulb tester in place?

Follow-up questions: What is the purpose of the dim bulb tester, and how does it work?

Yes. With it in place.
Attached is a rough sketch of what I’ve built and have with 100w incandescent bulb
The reason is because even tho I can turn the voltage up with a variac, if there’s an issue, I can “see” any problems AND, it works like a resistor ( the 100w bulb) and helps protect the circuit by dropping the current if there’s an issue.

[JMPGuitars]

dropping the current if there’s an issue.

The bulb acts as a current limiter. It's useful to find faults like transformer shorts, but you can't properly test current or voltage in the amp with the bulb tester connected to the amp. The wattage of the bulb has to be chosen based on the wattage of the amp. 100W is usually too high. You should do more reading on the subject...or unplug the bulb tester and move on, as it seems you've ruled out shorts at this point. Go ahead and use the variac to be safe, and then set ideal AC voltage before testing voltages again.

[foreverstrung]

The bulb acts as a current limiter. It's useful to find faults like transformer shorts, but you can't properly test current or voltage in the amp with the bulb tester connected to the amp. The wattage of the bulb has to be chosen based on the wattage of the amp. 100W is usually too high. You should do more reading on the subject...or unplug the bulb tester and move on, as it seems you've ruled out shorts at this point. Go ahead and use the variac to be safe, and then set ideal AC voltage before testing voltages again.

Right on. Good stuff Josh. That changed everything. Now with this removed my voltages are far more acceptable.
I’m attaching a voltage chart.
V4 was difficult to get a reading. I did the best I could. Amp was way warmed up, but I couldn’t get reliable readings. (I’m going to go thru this throughly in the morning)
Also note, I rewired the heaters so that they are all are 12a*7. I’ve marked the voltage chart to reflect this.
Another thing to note is that at 415 DCV at the plate for 70% dissipation I should be able to set the bias at 50.6 mA but I’m maxing out my bias trim pot at approx 44 mA
Appreciate all the help
Thx
Dave

[Bieworm]

OK that looks more like it! Good point Josh for ditching that lightbulb limiter!

About the unreliable voltage readings: install the tremolo switch jack! when the tremolo is not connected to ground it will wiggle. That wiggle will be messing with the power tube bias and will make the voltages bounce. You don't need a footswitch, just connect the hot switch from the tremolo jack input to ground (which you should anyway, so you can play without trem when no footswitch is connected)

Ok about the bias:
- start tweaking that voltage dropping resistor value. I always do that with a 100k resistor and another value in series until I get where I need to be (you can clearly see in my build that there are 2 resistors in series on that spot)
- 70% is too high to get proper tremolo. I bias mine between 50 and 60% dissipation. (IIRC around 42mA )

[foreverstrung]

About the unreliable voltage readings: install the tremolo switch jack! when the tremolo is not connected to ground it will wiggle. That wiggle will be messing with the power tube bias and will make the voltages bounce. You don't need a footswitch, just connect the hot switch from the tremolo jack input to ground (which you should anyway, so you can play without trem when no footswitch is connected)

This worked like a charm

Ok about the bias:
- start tweaking that voltage dropping resistor value. I always do that with a 100k resistor and another value in series until I get where I need to be (you can clearly see in my build that there are 2 resistors in series on that spot)
- 70% is too high to get proper tremolo. I bias mine between 50 and 60% dissipation. (IIRC around 42mA

A 180k resistor put this right.
Originally I had a 160k and I used a 100k first as suggested and it took the bias in the wrong direction. So I went to 180k and that seemed to balance everything out.
However, though the voltages may be acceptable, plugging in a guitar was not as successful.
I got a lot of noise with the NFT switch either engaged or not engaged,( I’m not sure which currently as I haven’t troubleshot anything yet. Just describing symptoms). With the switch in the current position it is quiet with volume engaged.
I do not have the reverb tank hooked up, but when I turned up the reverb it was a new, very unlovely sound.
When I turn up volume and master volume, I can hear the guitar thru the speaker but ever so barely and only with everything cranked.
My plan is to back off for a bit and when I come back, check again all my connections. (Not so much the solder joints though I know that’s important and I’ll keep an eye out, but rather that I’ve run everything correctly.)
After that, I was going to put the scope on it.
Appreciate your thoughts
Dave

[Bieworm]

Connect the reverb tank. That will quiet that reverb noise down. Put that pot on zero. Low sound level can point towards poor grounding or a poor connection. Wrong or forgotten coupling cap…

[Bieworm]

Can you also again post some detailed pics of the current situation?

When looking at your earlier pics I see some potential noise issues with the reverb circuit. (but hook up the tank te eliminate lots of noise already!)
- the wire from the return RCA jack to the grid of the recovery triode should be shielded wire and as short as possible.
- the green and black wires from the reverb transformer to the reverb input RCA jack are preferably flat to the chassis bottom. Certainly not flying. Don't forget that the reverb circuit in an amp is one of the most sensitive and potentially noisy parts of the amp.

GRIZZLY reverb comment.png

In that regard I'm experimenting at the moment with hardwiring the reverb cable directly to the circuit board.

[foreverstrung]

Can you also again post some detailed pics of the current situation?

When looking at your earlier pics I see some potential noise issues with the reverb circuit. (but hook up the tank te eliminate lots of noise already!)
- the wire from the return RCA jack to the grid of the recovery triode should be shielded wire and as short as possible.
- the green and black wires from the reverb transformer to the reverb input RCA jack are preferably flat to the chassis bottom. Certainly not flying. Don't forget that the reverb circuit in an amp is one of the most sensitive and potentially noisy parts of the amp.

I'll post some pics later this morning.
I found a mistake in the V3 reverb assembly with the 220k resistor going to the wrong pin. The layout I've got is not hi-def and I'm half blind, so I missed it. I'll dress this valve out much tighter. The pic you sent is extremely helpful. I am aware the dressing out the reverb valve/circuit is very important to noise.
Last night I hooked up the scope. I've already hit some hiccups right from the start following the signal to V1. I figured I'd start here this morning.
Uncover, discover, discard. (repair)

[foreverstrung]

Pics.
As I said, this Tefzel wire is a pain to dress out. I won’t use it again.
Also, I’ve got some sloppy connections. I know.
I still haven’t continued on from last night. These something interfering with the signal as early as V1 and reverb is still loud as sh-t with the tank hooked up. Obviously I’ll dress it all up tighter.
Thx
Dave

[foreverstrung]

More pics

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[Bieworm]

First try known good tubes in the reverb circuit, and everywhere else for that matter. You can solder the snot out of the amp while tubes can be the source of the noise. Believe me, happens all the time.. even with brand new tubes.
Another approach is to swap the tubes one by one with a known good one.but be aware that there can be more than one tube that’s bad. It happened to lecon my B15 build. All 3 preamp tubes were bad… brand new JJ’s!!!