Help Needed With Sluckey Pee Wee Hammond Build

[Vladymere]

Fellows (and Ladies),

I need help with my Sluckey Pee Wee Hammond build.  This is my first amp build or amp conversion.

I have checked my build against the schematic many times and think I have it wired correctly.  I have changed tubes, checked voltages, grasping at straws I have even changed the speaker output coil.

The problem is that shortly after power on the speaker goes into a maximum screech with a tremendous bass as well.  Loud enough to hurt my ears and I am partially deaf.

Voltages, measured with a Fluke 75 DVM and viewed on a Tecktronix 475 oscilloscope, are as follows, V3 (12AX7)is removed:
A – 311.7 VDC with some ripple
B – 298 VDC and clean
C – 290 VDC and clean
D – 288.6 VDC and clean
V1 pin 1 – -.02 VDC to +.02 VDC
V1 pin 2 – +2.06 VDC
V1 pin 3 – +299.6 VDC
V1 pin 4 – 3.28 VAC
V1 pin 5 – 3.28 VAC
V1 pin 6 – +313 VDC
V1 pin 7 – +9.27 VDC
V1 pin 8 – 4.49 VAC, -3.2 VDC
V1 pin 9 - +222.1 VDC

I have attached three photos to this post.
The first is a photo of my build.  ( I know the point to point wiring looks like a dog’s mess)
I do have the  68K ohm resistor from the input jack to V3-A pin 2 disconnected as part of the troubleshooting I was doing.
 

The second photo is of what I believe the problem to be.  V1 pin 1 has a small signal on it, even with the 12AX7 tube removed.  This signal is not seen on the 12AX7 tube pin 6.
Scope settings: .2 VDC per division,  2ms per division, ground is at first division from bottom.

The third photo is what is seen on V1 pin 6, the output to the speaker jack.
This is the amplified signal from the V1pin 1 input that ends up amplified to be this signal on V1 pin 6 (and inverted on V2 pin 6)
Scope settings: ground is at 1 division from the bottom, 100VDC per division, 1MS per division.
 
V3 pin 6 is a clean signal, the voltage is high at +284.5 VDC as the 12AX7 is not installed yet V1 pin 1 has a small signal on it.

Where is this signal in v 1 pin 1 coming from?

Thank you,

Eric aka Vlad (a monkey with a soldering iron and the burns to prove it)

[mountainhick]

Push pull amp right?

First try swap the output transformer primary wires to the other power tubes

The scoped waveforms are a bit concerning. Check all wiring into that tube, check plate and cathode resistors, check resitances at input jack(s).

[Vladymere]

I should have included a copy of the schematic in my post.

Attached is a copy of the Sluckey Pee Wee Hammond amplifier.

Eric aka Vlad (a monkey with a soldering iron and the burns to prove it)

[Vladymere]

mountainhick,

Yes, a push-pull amp.

Sunday I will try your suggestion of swapping the output coil primary leads although I have replaced the output coil with an identical Hammond model.

Eric aka Vlad

[dogburn]

Yeah, first thing to check is switching the output transformer primary wires. I've built two push-pull amps and both screeched like a banshee at first, and switching the wires fixed the problem right away.

[Vladymere]

Color me impressed.  I switched the output coil primary leads between the two power tubes and the screeching stopped.  I must say I don't understand why.

This is my first foray into tube amplifiers and there is a lot about there circuits I don't understand.  When I worked in electronics it was with TTL logic circuits in computer controlled mechanical devices.  POS terminals (cash registers), high speed check sorters , remittance processing equipment and such.  The computer controllers where initially 15" rack mount mini computers with 48K of magnetic core memory.  Hard drives where the size of washing machines with 10 megabyte removable platter assemblies.  Later devices where run on DEC PDP11 systems.  Data storage was also reel to reel tape drives.  We also had proprietary terminals that ran on MS DOS.

This Pee Wee Hammond amp does seem to work well now.  I did play around with different 12AX7 tubes to find one that wouldn't   provide issues at maximum volume and bass.

Mountain hickok, dogburn, thankyou fot your advice.

Vlad

[mountainhick]

Color me impressed.  I switched the output coil primary leads between the two power tubes and the screeching stopped.  I must say I don't understand why.

It is feedback. Look at the schematic at the negative feedback from the speaker. When in correct negative polarity, the ac signal being fed back to the V1a stage in that schematic, is the opposite polarity to the signal coming from V3B, so it subtracts from the V3B.

Now consider that if the output transformer primary leads are attached to the opposite power tubes. This inverts the audio signal, and the negative feedback becomes positive at the transormer outputwhere it reaches V1A. Positive feedback creates a squeal at the frequencies that loop through

[AlNewman]

Color me impressed.  I switched the output coil primary leads between the two power tubes and the screeching stopped.  I must say I don't understand why.

Vlad

Long story short, you changed positive feedback into negative feedback.

[Vladymere]

Mountain hickok,

Thank you for the explanation of the negative feedback loop.  I had to go find an Uncle Doug video to better understand how it works.

Vlad