Robinette Type 3 MV on AB165

[angelodp]

Hi, Just wondering about attempting the Robinette Type 3 MV on an AB165. I am seeing -48 volts on each 220k. Is this a dangerous idea?

A

[sluckey]

no

[angelodp]

No fear of shock with that voltage?

[pompeiisneaks]

Not sure what you mean? The pot's not connected to the lugs... the dial part is not connected to them, and 48V isn't much anyway.

The voltage stays on the lugs and wiper of the pot itself.

It's not present when someone uses it.

~Phil

[thetragichero]

good for Rob getting credit for this

[angelodp]

Very good, I'll give it a go before I try the 250k gang pot version. Any reviews on this version as being effective verses the LarMar?

Thanks All

[Ten Over]

good for Rob getting credit for this

I'm not sure if you're serious or just taking a sarcastic jab at Rob.

A lot of people seem to know about this MV from something called the Trainwreck Pages, but this MV has been around far longer than Trainwreck has (Trainwreck began in the early eighties).

This is a particularly dismal MV with its low frequency roll-off and frequency doubling.

[romberg]

Very good, I'll give it a go before I try the 250k gang pot version. Any reviews on this version as being effective verses the LarMar?

Go ahead and order the dual ganged pot now. I've tried both the LarMar and this version in the same amp. The LarMar is vastly better sounding. Especially at lower volume settings.

I think the reason this type of MV does not sound very good is that it counts on each side of the phase inverter output being exactly identical (just out of phase). If this were true then it probably would sound ok. However no phase inverter really does output identical but out of phase wave forms. And because they are not identical, then this MV does not equally cancel out the signal when the resistance is lowered. The effect is not very pleasant on the ears.

Mike

P.S. However it does sound kinda ok with a very heavily distorted grungy/trashy tone. It is just sorta a one trick pony and an acquired taste .

[Bergheim]

Very good, I'll give it a go before I try the 250k gang pot version. Any reviews on this version as being effective verses the LarMar?

I think the reason this type of MV does not sound very good is that it counts on each side of the phase inverter output being exactly identical (just out of phase). If this were true then it probably would sound ok. However no phase inverter really does output identical but out of phase wave forms. And because they are not identical, then this MV does not equally cancel out the signal when the resistance is lowered. The effect is not very pleasant on the ears.

Mike

P.S. However it does sound kinda ok with a very heavily distorted grungy/trashy tone. It is just sorta a one trick pony and an acquired taste .

I totally agree with Mike here. My taste generally tips in favor for nastier voiced amps than smoother lead tones, but the effect of the Type 3 mv on bedroom volume is just really weird. Gives a raspy distortion that initially led me to believe there was something wrong with the amp. I have a thread on this forum where I have scoped just what Mike describes regarding the PI outputs imperfectly cancelling each other and causing weird frequency-doubling and what not.
I guess the Type 3 is best suited as a simple retrofitted MV in smaller amps like a 5e3 where you don't need to turn the volume that far down to get bedroom volumes.

[thetragichero]

I'm not sure if you're serious or just taking a sarcastic jab at Rob.

neither. it's funny how things end up attributed to various folks across the years. i know it from the Trainwreck pages

[pdf64]

Over on MEF it was identified that a type 3 can result in a common mode voltage, that generally acts to increase power tube dissipation.
I’ve not yet verified that.
Previously, my feeling was that they can sound ok as drawn by KF, ie after a 12AX7 LTP.
However they sound bad with a Fender type 12AT7 LTP.
And they’re totally unsuited to a cathodyne (why put a master vol after a cathodyne anyway, whatever the type ).

[Ten Over]

Over on MEF it was identified that a type 3 can result in a common mode voltage, that generally acts to increase power tube dissipation.
I’ve not yet verified that.
Previously, my feeling was that they can sound ok as drawn by KF, ie after a 12AX7 LTP.
However they sound bad with a Fender type 12AT7 LTP.
And they’re totally unsuited to a cathodyne (why put a master vol after a cathodyne anyway, whatever the type ).

The 12AX7 LTP does the frequency doubling thing with a #3 MV, so it isn't going to sound good when turned down. Both cut low frequencies as they are turned down, being the result of R decreasing in the CR equation.

Yes, a #3 MV is totally unsuited on a cathodyne.

[angelodp]

Ok, I can confirm the raspy quality. It is a bit unsophisticated OD tone, but in some circumstances maybe useful. I do have the dual gang 250k on the way. BTW it seems that my amp is more in the AA864 camp. It was messed with long ago, not by me, and I have worked at bringing it back to spec. It does have the 220ks like the AA864.

I suppose I will have to remove those 220k Rs when implementing the LarMar

Appreciate the responses.

A

[angelodp]

I have completed the (reversible) conversation of the 67 AB165 to AA864 on the PI to power section, along with a Midpot conversion, NFB control, and PPIMV, and recap complete. Its pretty darn nice with bedroom OD if I want and 50 watts of goodness for the wide open moments. Quiet as a mouse.

Thanks All

A