1.5W top boost-style amp

[oxbow_lake]

Thanks again to you all for your comments and advice.

I think my next step is to buy another OT and give it a try. If that doesn't work then simplifying the power supply may be worthwhile, though pushing the screen voltage that high concerns me!

As I see it the output pentodes are problematic, with a transconductance of 13 mA/V or 13000 umhos, 200k plate resistance, 2 Volts p-p drives them fully open. They would be great as 1st stage voltage amplifiers.

Problematic in that the PI output is impractically high for them? Rob Robinette discusses this issue with his Bassman Micro circuit (using push-pull EF80s), mentioning that the master volume really becomes crucial for not just blasting the output tubes. I'm okay with this solution for now. I will admit I don't really understand how the plate resistance plays into output tube selection, and how it relates to OT impedance. Is there a rule of thumb for that?

Along those lines, I'd be open to suggestions for output pentodes that are ~3W and cheap. I saw that the 6AB8 has a similar maximum power, and the plate resistance is very small, so that's a good one to remember. I also see you can get Mullards for $9 which is not as cheap as 6BN11s but not bad. Generally, what are ideal values for transconductance and plate resistance? Presumably a cutoff voltage more like an EL84 would be better than 2Vpp.

A last question: I took out the screen grid resistors, but I am concerned that having the large filter cap directly feeding the screens might still allow excessive current draw?

[chumchai]

I think my next step is to buy another OT and give it a try. If that doesn't work then simplifying the power supply may be worthwhile, though pushing the screen voltage that high concerns me!

Don't buy an OT yet. Unless you shorted the primary to GND for two hours, I don't think the OT is blown. Test it first. Google is your friend. If you are a newbie to tube amplifier building, many things can go wrong, and you wouldn't be the first one. Just keep going and gain knowledge as you go.

Problematic in that the PI output is impractically high for them? Rob Robinette discusses this issue with his Bassman Micro circuit (using push-pull EF80s), mentioning that the master volume really becomes crucial for not just blasting the output tubes. I'm okay with this solution for now. I will admit I don't really understand how the plate resistance plays into output tube selection, and how it relates to OT impedance. Is there a rule of thumb for that?

Let me clarify. To me problematic are TV, transmitter, horizontal deflection, UHF radar and special purposed tubes that are repurposed for audio use. But that is just my opinion, other my disagree and I will not dispute it.
Just look at the datasheets of popular output tubes for comparison.

Along those lines, I'd be open to suggestions for output pentodes that are ~3W and cheap. I saw that the 6AB8 has a similar maximum power, and the plate resistance is very small, so that's a good one to remember. I also see you can get Mullards for $9 which is not as cheap as 6BN11s but not bad. Generally, what are ideal values for transconductance and plate resistance? Presumably a cutoff voltage more like an EL84 would be better than 2Vpp.

Let me correct myself. Transconductance is not the main issue here but the grid/plate characteristics. Just look at the datasheets of one of the best true audio pentodes of the past, the EL12. https://frank.pocnet.net/sheets/076/e/EL12.pdf
It has even higher transconductance of 15mA/V but a bias of about 12V and 30k plate resistance. BTW, you can still order them in Germany as EL12N. Nowadays there are no cheap audio tubes unless you look at Russian tubes shipped from Ukraine (Ebay).
How much did you pay for ECC83's? Some examples: 6P1P (6AQ5?) output tetrode, 6F3P(6BM8) triode-pentode, 6P6S (6V6) and a little known (sleeper) 12W 6P18P output pentode. You can also look at AA5 radio output tubes, but these are usually low voltage output tubes.

A last question: I took out the screen grid resistors, but I am concerned that having the large filter cap directly feeding the screens might still allow excessive current draw?

That is a legitimate concern. I would reduce the cap value to 0.1uF -0.22uF or put another resistor like 470R after the 50uF cap and 0.1uF-0.22uF at the screen grids. There are many schematics on this forum to study and try.

[bepone]

Ok. The first image is the signal at each grid. The second image is the anode voltage (AC coupled to get rid of the 280VDC) at each plate. Looks pretty nasty.

The purple text is hard to read in the photo, but the voltage scale is 1V/div for both the grids image and 5V/div for both channels in the anode image.

FWIW the traces look the same if I swap out for a different 6BN11.

i see that you have very small voltage developed on primary which is indicating almost short circuit on anodes like i suspected before.
1. remove completelly output transformer from Ub+ and anodes
2. connect 11k+11k 2W resistor in series , connect middle connection to Ub+ and endings to anodes,
3. measure again ocilograms on anodes,

in case you have 100Vpp or more , your problem is output transformer
in case you have some small voltage again on anodes, your problem are output tubes

all the other actions in this moment are not important

[bepone]

I think my next step is to buy another OT and give it a try.

Along those lines, I'd be open to suggestions for output pentodes that are ~3W and cheap. I

EL91, EL95,
piemme elektra for OT

[oxbow_lake]

1. remove completelly output transformer from Ub+ and anodes
2. connect 11k+11k 2W resistor in series , connect middle connection to Ub+ and endings to anodes,
3. measure again ocilograms on anodes,

in case you have 100Vpp or more , your problem is output transformer

Yep, this is what I did, the oscillograms are in a previous post and show a full 200+ Vpp swing on the anodes using a 15k load. So I agree this suggests that the OT may indeed be faulty.

Don't buy an OT yet. Unless you shorted the primary to GND for two hours, I don't think the OT is blown. Test it first.

I have actually been in touch with Hammond about ways that I can test it. The initiall response was to use an industrial-strength signal generator to a) check the maximum current at magnetic saturation, and if that's normal then b) apply a smaller AC voltage to the secondaries and measure the primary voltage generated on either side of the CT. But, since I don't have a signal generator that can put out enough power to do it, they asked me to at least measure the DCR of the primary and secondary, which I did, and which seems low on the primary but I am still waiting to hear back from them as to whether or not this is conclusive enough. I did do the second test, and got dissimilar voltages on either half of the primary, but again my signal generator was struggling so I'm not sure it's valid. So, I am holding off on buying the replacement until the friendly guy at Hammond gets back to me, but I *am* suspicious given that swapping the OT out for load resistors yielded the expected behavior. We'll see!

[martin manning]

This is what you did a week ago, no? Not saturated, but you got about half the expected impedance ratio. BTW you can use the heater voltage from the PT for the signal.

[chumchai]

Try to power the OT secondary winding from half of the filament transformer (3.15V). Measure exact voltages on primary/secondary. Knowing the winding ratio, one may evaluate the condition of the OT primary winding. Connect some load, like 2 x 47k on the primary.
Edit: Martin already preceded me on that one.

[martin manning]

Here's another test you can try that might reveal an internal short: https://ampgarage.com/forum/viewtopic.p ... 11#p445611

[oxbow_lake]

Sure, doesn't hurt to try.

For general knowledge in case anyone else finds themselves in my situation, the Hammond guy's precise instructions were to
first check for a short in the primary using a130V 400Hz signal applied between Blue and Brown and all others open. Current should be less than 15mA max, and is usually around 3mA.

That part I can't really do with the equipment I have.

After that, the suggested protocol was to apply 1.0V between the whole of the secondary Black 1 to White 6, which should yield ~32mA at 60 Hz. The outputs on the primary are meant to be ~9.4V each blue to red and red to brown.

So okay, I suppose I can try using half the filament supply as an approximation. I'll let you know what I get! Thanks for the suggestion.

[oxbow_lake]

I hooked up half the filament supply to the secondaries (black-white) with all the tubes pulled and no connection to ground, and hooked up a 100k resistor between primary leads (blue-brown). Got ~3.6Vac. Current in the secondary coil was almost 1A. Voltage from blue to red was 13V, voltage from brown to red was 26V.

I think my OT is bad.

[bepone]

sometimes i found new OT faulty put on the market, it is speed of manufacturing theese days, and not 100% testing in the batch

[chumchai]

That is odd. Exactly 2:1. Colors messed up? What is the voltage Brn-Blue?

[TUBEDUDE]

I hooked up half the filament supply to the secondaries (black-white) with all the tubes pulled and no connection to ground, and hooked up a 100k resistor between primary leads (blue-brown). Got ~3.6Vac. Current in the secondary coil was almost 1A. Voltage from blue to red was 13V, voltage from brown to red was 26V.

I think my OT is bad.

Looks like it's miswired. Try using the Blue as the center tap.

[martin manning]

Rather than guess, maybe use a DMM to see if you can find out which leads are on which side and what order they are in on the coils, same as you would do for any unknown transformer.

[oxbow_lake]

With nothing hooked up to the OT, I get the following resistances (in Ohms):
Primary:
brown-blue 238
brown-red 174
red-blue 63
Secondary:
black-orange 0.2
black-green 0.4
black-yellow 0.6
black-violet 0.7
black-white 0.8

Going from any of the primary leads to any of the secondary leads gives an open circuit. Apart from the brown-blue resistance being closer to 200R than the 300 it "should" be per Hammond, this looks pretty reasonable and like the colors are all labeled correctly.

Hooked up half of the heater xformer again to get 3.2Vac, got the following voltages on the primary taps:
brown-blue 36.7Vac
brown-red 24.5
red-blue 12.5

I did not have the 100k load resistor hooked up to the primary this time, just bare leads.

So to me this doesn't really look like the leads are miswired, though I agree the 2:1 voltage ratio is suspicious. On the other hand it's late and I'm tired, so maybe I'm being dumb. Haven't heard back from Hammond yet.