Multiple issues with new build.

[Shadrock2]

So I've put together an amp mostly like the schematic shown. With the following changes:

1. Reversed the polarity of Zener diodes in the Bias circuit.
Seems like they should have worked the way they are shown but they only had forward bias voltage drop that way. (0.7V ea.) Figured that out with the light bulb in series with power and no tubes except rectifier.

2. Substituted 10K resistor to ground from bias pots.

3. Added a 4 ohm 5W resistor in series with the primary leg (120V) of the Power transformer.
All voltages still to high. I am using an old PT with primary rated for 115V. My power has ranged from 119V to 124V.
The 5V and 6.3V heaters as well as the B+ were all to high and this helped get that closer to spec.
Are there any long term negative effects to doing this?

4. B+ dropping resistor between PI and V2b is 10K

So I have noted that the B+ voltage varies quite a lot with bias adjustment now. Not really a big problem but requires a little back and forth.

My B+ with 24ma current on Output Tubes is 415V on plates and 405-6V on Screens. PI supply volts about 380V.

So I got signal through the amp to my test speaker with my function generator. So I tried to play through it.

With no negative feedback I could play with the volume on 1 to 2 and it would work but sounds like a transistor radio. If I turned up past 2 the amp Motorboats. If I switch to the low negative feedback I can still play but now if I turn up it squeals. If switch to more negative feedback it squeals continuously.

So to start with it looks like I have to reverse the polarity of my Output transformer. Since the transformer has multiple speaker impedance taps I can't just reverse the 16 ohm and ground speaker leads. (Right?) So... the only thing I can do is switch the plate leads on the Output Tubes? Is that correct? Is there anyway tell the correct polarity before you hook up the OPT?

So the Motor-boating is also going to be an issue. Do you think a polarity change will cure that as well? Or, will that require more capacitors between stages? Recommendations?

Finally, How do I tell if I have an ultrasonic signal at speaker output?

[Shadrock2]

I have a layout drawing also.

And one more change to schemo. There is an 820 ohm grid stopper on the PI input.

[loudthud]

I tried building a TW Express on an old Traynor chassis. On that chassis the output transformer leads come through the chassis between the board and the pots. Bad idea. It really wanted to oscillate. Had to shield the leads from the transformer to make it stop.

Your schematic does not show a ground at the output transformer secondary. Is that a ground at the terminal strip next to one of the bias pots?

[Tom Phillips]

...With no negative feedback I could play with the volume on 1 to 2 and it would work but sounds like a transistor radio. If I turned up past 2 the amp Motorboats. If I switch to the low negative feedback I can still play but now if I turn up it squeals. If switch to more negative feedback it squeals continuously.

First, if you are going to use negative global feedback at all, you need to add a ground reference to your OT secondary. JPB asked about that ground too. Without the ground you have no reference for the feedback signal and all manner of bad things can happen.
Another comment regarding the amp sounding “like a transistor radio.” I assume by that you mean that the sound is thin and tinny. If so, then there is some other issue going on because lack of feedback will not cause that symptom.

...So to start with it looks like I have to reverse the polarity of my Output transformer. Since the transformer has multiple speaker impedance taps I can't just reverse the 16 ohm and ground speaker leads. (Right?) So... the only thing I can do is switch the plate leads on the Output Tubes? Is that correct? Is there anyway tell the correct polarity before you hook up the OPT?...

First fix the ground reference at the OT secondary. Then test the feedback phase by playing a sustained note and switch the feedback on and off. If the amp is louder with the feedback switch OFF then your negative feedback polarity is correct. If not then, yes, you need to reverse the plate leads.

Just work through the problems one step at a time.

Cheers,
Tom

[g1]

Any one care to comment on the bias circuit, and why he experienced the following?

1. Reversed the polarity of Zener diodes in the Bias circuit.
Seems like they should have worked the way they are shown but they only had forward bias voltage drop that way. (0.7V ea.) Figured that out with the light bulb in series with power and no tubes except rectifier.

[Shadrock2]

LT: "Your schematic does not show a ground at the output transformer secondary. Is that a ground at the terminal strip next to one of the bias pots?"

Yes the OPT secondary is grounded through the terminal lug shared with the bias ground. And... uh ... oops!? ... I neglected that on my schematic.

And... I may have neglected to to consult you guys first about the OPT leads. That is going to be hard to fix.

Will try to see which way is louder with or without feedback.

[g1]

As shown, no matter which way the zeners end up being oriented, that bias circuit does have a rectification stage.

I think you mean the bias circuit does NOT have a rectifier stage? That's what I thought, nor does it have an AC source. I don't know how he could measure a negative DC voltage at the power tube grids?

And... I may have neglected to to consult you guys first about the OPT leads. That is going to be hard to fix.

Do you mean hard fix because the OT leads length prevents swapping at the power tube sockets? Other than that, it should not be hard.
If that is the case though, you should be able to swap the power tube inputs from the PI.

[Tom Phillips]

I think you mean the bias circuit does NOT have a rectifier stage? That's what I thought, nor does it have an AC source. I don't know how he could measure a negative DC voltage at the power tube grids?...

Yep. The circuit as drawn couldn't produce a bias voltage. The whole loop of the 220/5W resistor, the zeners and the 100uF/100V cap to the left of the PT is effectively not in the circuit because both the "input" and "output" of that loop are grounded. We will need to hear from Shadrock2 about the origin of the schematic and what the intent was.

[Shadrock2]

I think you mean the bias circuit does NOT have a rectifier stage? That's what I thought, nor does it have an AC source. I don't know how he could measure a negative DC voltage at the power tube grids?


Do you mean hard fix because the OT leads length prevents swapping at the power tube sockets? Other than that, it should not be hard.
If that is the case though, you should be able to swap the power tube inputs from the PI.

So first of all I clearly don't understand how to do "quotes" here. I can only quote the entire message if I select [REPLY WITH QUOTE]. That doesn't even address quoting different posters. What syntax or procedure do I use to select only the line I want and how do I include multiple posters in one message.

[Shadrock2]

Forget the Zeners. The cathodes point towards ground as shown. I apparently can't see. The reference for the bias circuit is here:
What is Back-Biasing?. Think of it like... all the current from the B+ has to flow back to the centertap of PT. If you fix the ground upstream from that then the centertap is negative with respect to ground.

Ok... so tried to play with and without feedback and volumes were essentially the same but volume is real low cause with feedback it whistles. If I turn the volume up a little more I can just hear a tone at the top of the audio spectrum. With or without feedback if the volume is turned up it goes put...put...put, probably at about 5 cps, which I believe is classic motor boating.

And I meant it is going to be hard to relocate the output transformer so that the leads don't run out from under the turret board. Live and learn I guess.

The circuit I drew myself and is my own design... more or less. My thinking was that I could put a lot of gain up front and have the other stages more for tone shaping.

[Enzo]

Quotes are easy. Select the passage you want to quote, say the first part of your last paragraph above, left click your mouse to highlight the passage, then right click and chose COPY. Now it is on your clip board.

In the quick reply box now, right click your mouse and chose PASTE. That will place a copy of the passage in the reply, like this:

The circuit I drew myself and is my own design... more or less.

Now look at the tool bar along the top of the reply box. The last thing on the right is a little thought balloon like in a comic strip. That is the quote button. So now that your quoted passage is on the screen, mouse across it to highlight it again, then just mouse up to the quote button and click it. Et voila, your passage is now in a quote box. Before you post, in the reply box it will just look like [quote] before and after the passage, but once posted it will show in the little quote box thingie.

Thus:

The circuit I drew myself and is my own design... more or less.

The advanced reply box works the same, but there are just more things to look through to find the quote button

You can also start by hitting the quote button and two sets of [quote] will appear next to each other. You can then PASTE your passage between them, but I find the other method easier for my clumsy fingers.

Takes a lot longer to explain than to do.

[Dave H]

I think you mean the bias circuit does NOT have a rectifier stage? That's what I thought, nor does it have an AC source. I don't know how he could measure a negative DC voltage at the power tube grids?

The negative voltage is produced by the amplifier operating current having to travel from ground through the zeners and 220R to get back to the centre tap of the PT. As this circuit is effectively adjustable cathode bias I think I'd prefer to simplify it and just use normal cathode bias but with a resistor and cap for each tube. It will balance well enough like that.

Edit:
I wonder if the motorboating could be caused by the quirky bias circuit? I know it should work but I'm not too keen on the main filter caps not being connected directly to ground.

[Shadrock2]

Thought Balloon huh... Thanx E

Now look at the tool bar along the top of the reply box. The last thing on the right is a little thought balloon like in a comic strip.

Thank you Enzo!!!!!!!!!!!!!!!!

I wonder if the motorboating could be caused by the quirky bias circuit? I know it should work but I'm not too keen on the main filter caps not being connected directly to ground.

That's Easy enough to check. I can change the negative cap wire to ground. Though its hooked up this way it is to protect the Zeners from the initial charging surge. Looking at the spec sheet for Zeners they indicate that they can withstand a substantial surge for a few milliseconds.

[Tom Phillips]

Thought Balloon huh...

Regarding the quote feature I like to use the "Reply with quote" button at the bottom right corner of the post I wish to quote.
One click and the whole post is presented to you in a reply window. Then you can edit the quote to the relevant section of the post that you are discussing (as I did above) and add your comment outside the quote begin & end codes which are shown in square brackets. This method is very fast. It also shows the source of the quote.
Cheers,
Tom

[Dave H]

That's Easy enough to check. I can change the negative cap wire to ground.

If you do that watch out for extra ripple on the bias supply caused by the filter cap charging current pulses through the zeners and 220R. You may have to increase the value of the 100u cap between -40v and ground.