Hammond AO-33-3 treble OT- pr 6BQ5s please ID extra leads

[chopstuck]

I thought that these extra orange and white leads were UL taps at first but I then noticed they were attached to pin 3, (the cathode) of the EL84.
They have a black center tap too.

The OT was a treble OT for a three channel AO-33-3 amp chassis found in a par 40.

I tried just insulating these small leads, tying them off and using the main, larger leads. Well in this old AO-43 reverb chassis it just squeals.
The conjugate filter worked and it sounded good but now it fries the 10k 1W resistor. Even the 3 watt smokes right away.

Why would the extra leads connect to pin 3 cathode ?

I have 4 of these and I don't know if I can interchange them with a stock AO-43 reverb OT

[chopstuck]

I found the schematic and see some sort of feedback to the cathode through this tap.
Is this workable without this arrangement? Or is it better to connect the cathode through this winding ?

I see just a rewire and maybe 2 caps to make it work.

Like I said I have four of these and would like to be able to use them.

http://www.captain-foldback.com/Hammond ... QR40_1.gif

[rdjones]

That looks like a split load or distributed load transformer.
Part of the winding is above the tube, part below.
The cathode resistor and bypass is shown drawn next to the speaker which is a bit misleading.

I'd be curious about the turns ratios, which might determine if the transformer could be used in a different type of circuit.

Here's a redrawn Quad II circuit which may be easier to understand with the two primaries shown next to each other.

rd

[chopstuck]

Just measured my transformer and got these results.

Primary to main secondary Turns Ratio 29 IR=29^2 or 841.

Assuming 8R secondary load. 8R x 841= 6728

So Primary Imp =6.7K

Looks good for pair of EL84's

Primary to smaller secondary TR 3.16 IR=3.16^2 or 10

Using the Primary Imp of 6.7k/10IR = 672R small winding secondary imp.
I'm interested in the function of this winding if you have any further ideas
I would like to hear them.

I ended up mounting the laydown OT vertically and it solved all my problems. Ugly but good sounding unit on my Spit clone.

I will NEVER do another AO-35 reverb amp . Terrible packaging for my so-so lead dress. Too much time involved.

[rdjones]

Just measured my transformer and got these results.

Primary to main secondary Turns Ratio 29 IR=29^2 or 841.

Assuming 8R secondary load. 8R x 841= 6728

So Primary Imp =6.7K

Looks good for pair of EL84's

Primary to smaller secondary TR 3.16 IR=3.16^2 or 10

Using the Primary Imp of 6.7k/10IR = 672R small winding secondary imp.
I'm interested in the function of this winding if you have any further ideas
I would like to hear them.

The "other winding" is actually part of the primary and is effectively in series with the main part of the primary.
It just happens to be on the other side of the tube connected between the cathode and ground through the cathode biasing resistor.
It provides negative feedback similar to an ultralinear design and is a low distortion HiFi topology.
Your measurements indicate it to be 10:1 (10%) of the total load impedance of approx 7500 ohms.
7.5k is a common load impedance for 2x6BQ5 in organ and HiFi amps.

You can wire these windings in series with main primary (making sure they are in phase) to use it as 7500:8.

Here's another drawing from "Valve and Transistor Audio Amplifiers" by John Linsley Hood showing this circuit in simplified single ended form.

reddog Steve

[Reeltarded]

Sorry so disruptive, but that is the insignia on the side of the UFO that probed me.

You crazy Egyptians and your oddball Sumarian friends.. tsk..

[dorrisant]

You can wire these windings in series with main primary (making sure they are in phase) to use it as 7500:8.

So how do you know which way would be in phase?

Tony

[Phil_S]

I'd run a low voltage test to see if it's in phase. Connect the two windings. Run low VAC through the transformer. You already know the turns ratios. See if the predicted = actual (or reasonably close).

You can also predict the out of phase voltage. When out of phase, the windings in series will subtract instead of add.

[dorrisant]

Thanks Phil!

Tony

[chopstuck]

A belated thank-you from me too Steve.

I have tried running one of these with the extra winding tied off and it really did not sound good, then died. I will give this series wiring a try too. I have a few of these to experiment with.

Marc

[rdjones]

A belated thank-you from me too Steve.

I have tried running one of these with the extra winding tied off and it really did not sound good, then died. I will give this series wiring a try too. I have a few of these to experiment with.

Marc

None of the schematics I found show any transformer lead color codes or markings.
As Phil points out, careful measurements would be needed to determine polarity.

I found this Hammond schem that shows a center tapped cathode winding, with the center tap connected to a cathode bias resistor/cap.
It may help shed some light on the arrangement.
With the CT'd (otherwise common) tertiary winding there should be 2 possible combinations.
Hopefully this helps answer Tony's question.

rd

[dorrisant]

Thanks rd.. I have an AO33 and I will try to do some low voltage checks tomorrow and post a color code.

I posted a picture of mine above... Is it visible? Just curious. My phone does weird things sometimes.

Tony

[dorrisant]

Ok... Same treble OT from AO 33:

Main PRI - YELLOW and BLUE
Main PRI CT -RED
Small PRI - WHITE and ORANGE
Small PRI CT - BLACK
SEC 8ohm - BROWN and GREEN


I measured the same as chopstuck on the Main PRI.

Added the Small PRI in series with Main PRI.
13.85vac applied to ORANGE and BLUE with YELLOW and WHITE tied together.
Measured 0.438vac on SEC.
Turns Ratio= 32 to 1
Impedance Ratio=1024 to 1
With 8ohm load PRI Impedance= 8.2k

With the two primaries out of phase (VAC applied to ORANGE and YELLOW with WHITE and BLUE tied together), PRI impedance was 5.2k.

I'm assuming to leave the BLACK Small PRI CT disconnected/capped if both primaries are connected in series. Right?

Now, what happens to the Main PRI in relation to the new taps? Wouldn't the CT be off center wired this way? What effects might we expect?

Tony

[chopstuck]

Is there any good use for the push pull 12BH7 reverb driver transformer in this chassis ? I have a couple of those and was wondering about a push pull 2 watt amp - looks like an 8ohm load.

Not to jack my own thread. he he

marc

[rdjones]

Ok... Same treble OT from AO 33:

Main PRI - YELLOW and BLUE
Main PRI CT -RED
Small PRI - WHITE and ORANGE
Small PRI CT - BLACK
SEC 8ohm - BROWN and GREEN


I measured the same as chopstuck on the Main PRI.

Added the Small PRI in series with Main PRI.
13.85vac applied to ORANGE and BLUE with YELLOW and WHITE tied together.
Measured 0.438vac on SEC.
Turns Ratio= 32 to 1
Impedance Ratio=1024 to 1
With 8ohm load PRI Impedance= 8.2k

With the two primaries out of phase (VAC applied to ORANGE and YELLOW with WHITE and BLUE tied together), PRI impedance was 5.2k.

I'm assuming to leave the BLACK Small PRI CT disconnected/capped if both primaries are connected in series. Right?

Now, what happens to the Main PRI in relation to the new taps? Wouldn't the CT be off center wired this way? What effects might we expect?

Tony

I don't think the third "tertiary" winding can be used in series with the primary, at least not on the high voltage side.
As you suspect it would be off center.

The 2 possible configurations (using the winding) would be in the cathode path, either out of phase as a feedback loop, or in phase to reinforce the primary.
The tertiary CT would be grounded in a fixed bias design, or go through the cathode bias resistor as shown on the Hammond schematic.
If you use the winding to reinforce the main primary you might need to add a traditional negative feedback loop.
Of course you could just insulate the leads and ignore the winding.

rd