Magnatone Custom M10

[TelRay]

please allow me to do some "brute-force-reverse-engineering" with no theoretical basis whatsoever, just what I have observed by measuring

this is with the intention of understanding the selection of a new transformer (if needed)

HIGH VOLTAGE: the two transformer secondary RED wires are delivering (measured value) 320 VAC which after the RECTIFIER end up in 213 VDC
I am assuming that to get 400 VDC required by the amp's schematic i would need a transformer that delivers 601 VAC (400 VDC * 320 VAC / 213 VDC)
as, by some reason (there surely is a good explanation), whatever AC voltage I send through the rectifier ends up being divided by 1.5 and transformed in DC voltage

if that is the case, the HAMMOND 370BX seems to be the closest (601.4 VAC)

https://www.hammfg.com/files/parts/pdf/370BX.pdf

BIAS VOLTAGE: the Hammond transformers I've seen, they all deliver 50 VAC and I am failing to see how 50 VAC are going to become -50 VDC

https://www.hammfg.com/electronics/t...rs/classic/300

in any case I am willing to understand who to do some "magic" with the diode arrangement to get the current transformer to do the job

[The Dude]

......Examples would be Ampeg V4 or Marshall JCM900 series, both using full-wave bridge and cap coupled type bias circuit.

^^^^That. Find one of those schematics and look at how bias and B+ are derived.

[Helmholtz]

With a full-wave two-phase rectifier (always requires a grounded CT) only one of the secondary halves is conducting at a time and the unloaded DC at the smoothing cap is around 1.4*VAC/2, VAC being the RMS voltage of the full secondary.

Using a fullbridge rectifier doubles the unloaded DC because the full winding is engaged all the time. No CT required.

The loaded DC depends on a number of influences.

BIAS VOLTAGE: the Hammond transformers I've seen, they all deliver 50 VAC and I am failing to see how 50 VAC are going to become -50 VDC

Rectified 50Vrms gives around 70VDC (roughly corresponding to the peak value of 50Vrms) at the smoothing cap. Voltage dividers are used to reduce to desired bias voltage. Polarity depends on the orientation of the diode.


http://www.valvewizard.co.uk/bridge.html

[TelRay]

thank guys all for your input, after re-reading your posts I am (almost!) confident I can start transforming the circuit from FULL WAVE to a BRIDGE configuration.

as always, let's first see if I got everything right "on paper" before heating up the soldering iron

In summary,

- I will be using the same 2 RED cables coming out of the TRANSFORMER secondary
- I WILL NOT be using the CENTER TAP of the TRANSFORMER secondary (RED / YELLOW wire) and I should isolate it (not connected to GROUND anymore)
- the (newly added) NEGATIVE wire that will come out of the BRIDGE RECTIFIER should be now connected to GROUND (this is the bit I want to triple check with you guys)

In form of a schematic, that is:

[Helmholtz]

Yes, approximately. And if you increase the cathode resistor to something like 220R to 270R/5W (best value has to be determined by experiment) with bypass cap and connect point F to ground you don't need a grid bias supply winding.

[g1]

I like the full cathode bias idea. I didn't realize earlier that it was already partial cathode bias. So it should not change the sound of the amp much I don't think.

And reminder that the bias tap wire of transformer also needs to be disconnected and insulated.

[TelRay]

thx for the last two posts, time I put all together as I understand now that both changes to the HV and BIAS supply need to happen together.

HV POWER SUPPLY
I implemented the FULL BRIDGE RECTIFIER as describe above. However, I forgot to mention (G1's comment made me realize) that when I disconnect the 0 V RED/YEL WIRE from the secondary this does not only affect the HV but also the BIAS voltage.



This means there is now no negative BIAS VOLTAGE going to the power tubes and I should expect no sound if I turn the amp on.

CATHODE BIAS
If I understand Herr Helmholtz right.

- "you don't need a grid bias supply winding". that means now the TRANSFORMER SECONDARY will look like this



- increase cathode resistor to 220R - 270R/5W
- add bypass cap
- connect point F to ground

these 3 modifications highlighted in YELLOW



QUESTIONS:
- can you confirm i got all this information right?
- how is the selection of the right CATHODE RESISTOR done? Would I need to measure VOLTAGE or CURRENT targeting a certain value?
- how is the bypass cap chosen (it'll be an electrolytic)?

thank you gentlemen!

[The Dude]

With cathode bias only, bias is set by the cathode resistor value only since there is no bias supply. Resistor value is chosen for the tube dissipation you are looking for. You measure the voltage drop across the resistor and use ohm's law to calculate currrent. This will give you some idea.

http://www.tedweber.com/webervst/tubes1/calcbias.htm

I'd leave the cathode cap value as is, although you may have to raise the voltage rating of the cap with a larger resistor. Make sure it's at least rated for the voltage present at the cathode.

One other correction: Your statement, "This means there is now no negative BIAS VOLTAGE going to the power tubes and I should expect no sound if I turn the amp on." is wrong. With no negative bias voltage tube current will be extremely high and likely red plate. There will be too much conduction rather than not enough.

[g1]

All correct except bypass cap polarity drawn reversed. I'll let someone else get into the math for the bypass cap value, I'd be inclined to copy from an established design I liked the sound of.

[TelRay]

...Resistor value is chosen for the tube dissipation you are looking for

According to the specifications of the 7189A tube, the Max Cathode Current is 72 mA. And with the link you've shared I have a rough idea of where to go.

I'd leave the cathode cap value as is, although you may have to raise the voltage rating of the cap with a larger resistor. Make sure it's at least rated for the voltage present at the cathode.

well... there's currently no cap across the cathode resistor (as per the original schematic)

One other correction: Your statement, "This means there is now no negative BIAS VOLTAGE going to the power tubes and I should expect no sound if I turn the amp on." is wrong. With no negative bias voltage tube current will be extremely high and likely red plate. There will be too much conduction rather than not enough.

in that case I might have fried a perfectly working pair of 7189A's

All correct except bypass cap polarity drawn reversed. I'll let someone else get into the math for the bypass cap value, I'd be inclined to copy from an established design I liked the sound of.

thank you mister. changed (edited above)!

[Helmholtz]

- how is the selection of the right CATHODE RESISTOR done? Would I need to measure VOLTAGE or CURRENT targeting a certain value?

Cathode biasing is different and allows for considerably higher dissipation at idle than with fixed bias, so forget the 70% rule. Reason is that the voltage drop across the cathode resistor (= bias voltage) increases with signal level while with fixed bias it stays constant/fixed.

The 7189 datasheet gives a design example for UL operation with Vp = 375V, Ip = 2 x 35mA, Rcath = 220R. From this the dissipation at idle is 375V*35mA = 13.1W or just below 100% (13.2W) for this tube.
As your plate voltage is higher than 375V you will need to reduce cathode current by using a higher value cathode resistor than 220R to stay below 100% dissipation.

- how is the bypass cap chosen (it'll be an electrolytic)?

Bypass capacitance influences the bass cutoff frequency. But it's never the cap alone but rather the RC product which determines the frequency response. So a higher value resistor cathode allows to use a lower value cathode cap for the same cutoff frequency. You may copy the RC value from other amps and calculate the desired C by dividing by your resistance. You can go as low as 4.7µF if you like. Or as high as 100µF. The cap value not only (weakly) affects bass response but also dynamics (bias recovery time).
Voltage rating 50V.

[TelRay]

richtig Herr Komissar!

so, it's starting to look like this



RESISTOR value to be adjusted to get dissipation just under 13.2W at each tube
CAPACITOR value to be chosen after fixing the RESISTOR VALUE

would it be safe to measure the tube dissipation without the by-pass capacitor to determine the RESISTOR VALUE?
after this step, calculate the needed BYPASS CAPACITOR value that will provide the same cut off frequency than a known existing circuit (can you suggest any? I know G1 had something in mind!)

thx!

[Helmholtz]

richtig Herr Komissar!

So you knew? BTW, my correct title is Diplom-Physiker.

RESISTOR value to be adjusted to get dissipation just under 13.2W at each tube
CAPACITOR value to be chosen after fixing the RESISTOR VALUE

would it be safe to measure the tube dissipation without the by-pass capacitor to determine the RESISTOR VALUE?
after this step, calculate the needed BYPASS CAPACITOR value that will provide the same cut off frequency than a known existing circuit

Correct, bypass cap has no influence on idle current.

[TelRay]

Dr Hr Kom-Dip-Phy

[Helmholtz]

Dr Hr Kom-Dip-Phy

Please stop it.