First question on my build

[J Luth]

The amp is based on a Fender blackface AA864 Bassman with a few additions and tweaks. I used the Bass channel circuitry only. It a single channel with Volume, Treble, Mid and Bass. 2 – 12AX7’s, 1 12AT7 in phase invert er and 4 – 6L6GC output tubes.


I attached a schematic of the bass amp I built. This is the first schematic I have ever drawn so hopefully it is done correctly. I have a problem with the build, When I built it I did not install the 100 ohm resistor to ground that that connects to the Phase inverter circuit shown on the schematic as not installed. The amp has been working fine without it. Once I saw that I forgot it I installed it and with it connected the amp has a noticeable hum and the volume gets extremely loud. The volume control can not be tuned above 1 ½ – 2 before it is over driving and distorting. I tried various other resistors just to see what would happen. Not much different until I put in a 4700 ohm resistor. The volume was still a slight bit louder than with no resistor but not much.

If anyone would have any idea what might be the cause, I would appreciate it.

Jim

[eschertron]

The 100 Ohm resistor is an essential part of the negative feedback circuit (NFB) that controls the gain of the power stage.

I recommend lifting either end of R18 (820 Ohm resistor) and reinstalling the 100 Ohm resistor. This breaks the feedback loop. Now you can test the loop.
Then, if making the connection on R18 makes the amp louder and more distorted, the NFB is wired out of phase, making it positive feedback. There are many discussions that mention this on the boards, but in a nutshell, the fix for that is to swap the primary leads on the OT. This brings the phase back to what the NFB needs to see on the secondary side of the OT.

...and Welcome to the place! There are many knowledgeable folks here that can help. Not sayin' if I'm one or not

[Chuck H]

Hello, and welcome (again) to the forum. I think I've got a bead on the issue, but you may not like it because it's going to complicate your project and possibly reveal another issue that needs trouble shooting.

When I built it I did not install the 100 ohm resistor to ground that that connects to the Phase inverter circuit shown on the schematic as not installed.

Does that mean there was nothing connected to ground at the tail of the phase inverter? As mentioned, that 100R could be part of the phase inverter circuit as well as the negative feedback circuit depending on what was grounded or not.

The amp has been working fine without it. Once I saw that I forgot it I installed it and with it connected the amp has a noticeable hum and the volume gets extremely loud. The volume control can not be tuned above 1 ½ – 2 before it is over driving and distorting.

It's possible that the amp is too quiet without it, rather than being too loud with it. The hum may be a different issue that you are simply able to hear with the phase inverter working correctly so the amp is producing correct power. The amp breaking up early on the volume knob could be a consequence of using a linear pot for the volume control or perhaps a bad or improper ground in the preamp (which may also explain the hum). Do you know the taper for your volume control pot? When that amp is working correctly, and with the correct taper for the volume pot I would expect it to start breaking up around 4 or 5 on the volume control with the bass guitar volume knob up. If it did not do this before installing the 100R resistor in in the PI tail that could indicate the amp was not driving the power tubes correctly due to low or improper phase inverter performance.

I tried various other resistors just to see what would happen. Not much different until I put in a 4700 ohm resistor. The volume was still a slight bit louder than with no resistor but not much.

This observation also backs my suspicion that the high volume in the other arrangement is correct and the low lower volume condition was not. If everything else is wired correctly that 4.7k resistor elevates the negative feedback VERY high. The feedback circuit is a voltage divider and relies on a ratio. The stock ratio is 8.2:1 with a 4.7k resistor in the place of the 100R the ratio is elevated to 8.2:47. Or, to put it another way, 47 times too much negative feedback! That would would pretty much squash drive to the power tubes and must detriment performance similar to when the phase iverter wasn't grounded.?. Anyway...

What sort of bench test gear do you have? We can test for some of this.

[J Luth]

escherton
I'll give your suggestion a try

[J Luth]

Hello, and welcome (again) to the forum. I think I've got a bead on the issue, but you may not like it because it's going to complicate your project and possibly reveal another issue that needs trouble shooting.


Does that mean there was nothing connected to ground at the tail of the phase inverter? As mentioned, that 100R could be part of the phase inverter circuit as well as the negative feedback circuit depending on what was grounded or not.

Yes, there was, is no ground at the phase inverter.


It's possible that the amp is too quiet without it, rather than being too loud with it. The hum may be a different issue that you are simply able to hear with the phase inverter working correctly so the amp is producing correct power. The amp breaking up early on the volume knob could be a consequence of using a linear pot for the volume control or perhaps a bad or improper ground in the preamp (which may also explain the hum). Do you know the taper for your volume control pot? When that amp is working correctly, and with the correct taper for the volume pot I would expect it to start breaking up around 4 or 5 on the volume control with the bass guitar volume knob up. If it did not do this before installing the 100R resistor in in the PI tail that could indicate the amp was not driving the power tubes correctly due to low or improper phase inverter performance.

Without the 100 ohm resistor the volume of the amp appears to be fine. I have used it a lot at gigs with full band and I never have to have the volume over 3 or 4. The amp gets quite loud and is still clean up to about 6. The volume pot is a 1 meg audio taper.


This observation also backs my suspicion that the high volume in the other arrangement is correct and the low lower volume condition was not. If everything else is wired correctly that 4.7k resistor elevates the negative feedback VERY high. The feedback circuit is a voltage divider and relies on a ratio. The stock ratio is 8.2:1 with a 4.7k resistor in the place of the 100R the ratio is elevated to 8.2:47. Or, to put it another way, 47 times too much negative feedback! That would would pretty much squash drive to the power tubes and must detriment performance similar to when the phase iverter wasn't grounded.?. Anyway...

Also, when I first put it together I had grounds in about 4 or 5 different locations. I have since changed them to star ground.

What sort of bench test gear do you have? We can test for some of this.

I don't have any real test gear, just a few volt meters and cap tester. As I stated earlier I am just a hobbyist but I am considering getting a scope, signal generator and some better meters.

I knew I should have taken all my fathers test gear when he offered it to me many years ago. He was heavily into electronics his entire life. He worked for H H Scott for what seemed forever.

Jim

[Chuck H]

I knew I should have taken all my fathers test gear when he offered it to me many years ago. He was heavily into electronics his entire life.

That stuff never has much resale value and always looks like obscure junk sitting around. My own bench looks like a bunch of useless old swap meet crap. I use the stuff all the time Hindsight is always 20/20. The good news is that if you ever really get the bug it's possible to outfit a bench with pretty much every bench tool you need to work on guitar amps for about three to four bills used. Oscilloscope, signal generator, light bulb limiter, solder station, and a decent DMM will do 99% of whatever you run across. But that's a discussion for another time/thread.

First, this suggested procedure requires you to work in a live, open chassis with potentially fatal voltages present. Please be safe and careful.

I'd suggest putting the 100R back in place. Then plug the amp in and turn it on. Notice if the hum is present with nothing plugged into the amp. If it's not excessive in this state keep the volume down and take DC voltage readings at all the tube pins except 4,5,9 on preamp tubes and 2,7 on the power tubes. Black probe to ground, red probe to pin. Please list the tubes V# like this:

V1
1)210
2)0.12
3)1.5
6)220
7)0.13
8)1.6

V2
1)etc...

Now, is hum the affected by the volume control? Tone controls? Then, with the amp on and humming (even if you have to turn up the volume a little to get hum), put the amp in standby and pull the first preamp tube. Take the amp out of standby. Is the hum still there? If yes pull the next preamp tube. Hum still there? etc. until the hum stops.

Then please power down and put the volume control on 5. Measure ohms from either end to the center tap and report.

If you run these tests we can probably answer to many questions and issues.

[J Luth]

Chuck H

I will try your suggestions and report back.
I do have a bulb limiter.
I know about working on the amp when hot, I do have a bit of experience with this but still on the learning curve.
I hate to say it but, when I was young, dumb and messing around many many years ago, I got an over 400v shock. It certainly did tingle a bit.

Jim

[Chuck H]

I hate to say it but, when I was young, dumb and messing around many many years ago, I got an over 400v shock.

Yeah, I did that.

It certainly did tingle a bit.

Yeah, it sure did.

[J Luth]

Chuck

Voltage readings

V1
1) 263
2) 0
3) 2.27
6) 264
7) 0
8) 2.2

V2
1) NOT USED
2) NOT USED
3) NOT USED
6) 272
7) 0 but the hum changes
8) 2.1

V3 PI
1) 251
2) 59 hum changes tone and lower
3) 95
6) 236
7) 63
8) 95

V4
3) 442
4) 442
5) -54
6) 442

V5
3) 442
4) 442
5) -54
6) 442

V6
3) 442
4) 442
5) -54
6) 442

V7
3) 442
4) 442
5) -54
6) 442
Volume, No change in hum until over ½ then it gets louder, no change with tone controls, hum only slightly lower with V1, V2 removed one at a time, gone with V3 PI pulled.

Volume pot marked 1MA
Across both outside term 1.2M
Turned ½ way up
From Center to one outside term .851
From center to other side .092

[J Luth]

Also tried this
Lifted one side of the 820 so it was out of the circuit and installed the 100. Amp hums and gets real loud fast.
Took voltage reading at the .1 cap that pin 7 from the PI is connected. got 63 VDC and hum changed slightly. I switched to AC on the DVM and got 114 VAC and with the prob connected the hum was gone and the volume dropped.

[J Luth]

eschertron

When you say switch the primary leads are you talking about the leads to the power tubes?

[J Luth]

Also wondering about the power trans wires.
It is a classic tone 40-18004

It is connect with the 2 - red wires one each to each side of the rectifier, red blue to the bias resistor, 2 - green one each to the pilot light with 100 ohm resistors on each side to ground, red yellow to chassis ground, orange to chassis ground black hot, white neutral.

[eschertron]

Also tried this
Lifted one side of the 820 so it was out of the circuit and installed the 100. Amp hums and gets real loud fast.
Took voltage reading at the .1 cap that pin 7 from the PI is connected. got 63 VDC and hum changed slightly. I switched to AC on the DVM and got 114 VAC and with the prob connected the hum was gone and the volume dropped.

Is this as loud as before when both the 100 and 820 R resistors were in place? Does making the 820 complete the circuit (I use jumpers in an open chassis for this kind of troubleshooting) cause the volume to go up or down?

Using a meter on the high-impedance grid inputs often gives odd results, rarely anything useful. Not even the voltage measurement. (The meter in parallel with the grid loads the circuit down)

When you say switch the primary leads are you talking about the leads to the power tubes?

Yes. But let's find out why the amp is "hummy" first.

Also wondering about the power trans wires.
It is a classic tone 40-18004

It is connect with the 2 - red wires one each to each side of the rectifier, red blue to the bias resistor, 2 - green one each to the pilot light with 100 ohm resistors on each side to ground, red yellow to chassis ground, orange to chassis ground black hot, white neutral.

Looks good from what I see on the spec sheet. Make sure that the red/yellow center tap wire goes as close to the negative pole on the first filter cap (reservoir cap) as possible. If it goes to the chassis, or anywhere else not at the filter, the currents that complete it's loop "drag around" all the voltages that may be present on what aught to be a pure ground. Read up on lead dress, especially ground leads, if you have questions. You'll find out more than I could possibly type in this thread

My first concern is that everything may be normal, and the amp is simply way louder than you'd expect when up close and personal. Make sure the speakers aren't in your ear when you test it out.

[Chuck H]

Well huh... Your DC voltages look right enough and your volume pot is definitely a log taper.

You don't need to reverse your output transformer leads.

Power transformer looks to be allocated correctly.

Are you certain you have 114VAC on pin 7 of the PI? No decimal in there? Because if you do that's really wrong.

By pulling tubes you've isolated the problem to the PI. The PI grids are at a very high impedance and it's not unusual to get some anomalous readings and amp behavior when touching them with meter probes. But silence and 114VAC is definitely wrong. Please double check that and go ahead and measure the filament VAC from each green secondary to ground.

[eschertron]

Do you have a impedance selector switch on your build? Is the position of that switch matching the speaker load?

Just as a tangent, I see this schem has the NFB tapped at the speaker. What do others think about putting that FB point on a set tap?