Old Ampro PP 6v6 center tap question

[goldstache]

Was converting my buddy’s Ampro satellite PP 6v6 power amp into a guitar ready plug and play deal.
It went well, but one thing stock to the amp had me pondering.
The power transformer HT center tap is floated from chassis ground with a big ol’ 330ohm resistor.
The amp is full wave rectified by a 5y3.


Never seen that before?!???


Anybody got any insight as to why?
Is it just an old school form of fusing?


Any thoughts?

[DuaneOh]

I've seen resistors on the HT center tap used for "Back Biasing", using a resistor like that would be a way to reduce the B+ voltage. You can read about back biasing here:
https://www.aikenamps.com/index.php/what-is-back-biasing

[goldstache]

Thanks so much! Oddly, it’s a cathode biased amp and chassis is zero volt for all ground references.
The point at which CT and float resistor meet, there are no other connections.


I wondered if it was a way to shrink B+ but never even took a voltage from the top of the resistor to ground.


Found it very unique and wondered why? HT reduction, noise combat?


Odd

[thetragichero]

sort of a lower tech version of zener(s) between center tap and ground?

[DuaneOh]

The only purpose I can think of for that resistor is to reduce the HT.

[goldstache]

Thanks y’all! Makes sense as it’s operating class A and bias current barely moves, so the reduction, though present, bias currents aren’t darting around during play.
First time for everything. Safe to say it’s best to take all voltages referenced  from the intersection of CT and float resistor?
For real world operating points?


Also, when safety grounding chassis is still the go to? All preamp/power amp grounds reference the chassis.

[Greg]

It is series resistance. You probably have about 24 volts or so across that resistor. The ground reférence must be the chassis. I've seen series resistance before reservoir caps in some vintage single-ended old home stereos, ususally about 220 ohms or so. Certainly not a fuse if it's a big resistor. Maybe that resistor is there to accomodate a larger input capacitance than permitted by the 5Y3 specs or to protect old ss rectifiers at turn-on.

[goldstache]

Thanks for your time. Yeah it’s likely 5w old part. When you mentioned before filters, I thought of the brute force Gibson EH-150 technique of lowering HT for plate and preamp via a big V divider.
Oddly reservoir is 30uF in this puppy. But your input does ring true as to why the heck they did the float. I think it was iron B+ and class A data sheet operating point prolly.

[Greg]

Well I made an educated guess but I don't know the original intention for sure. We could maybe pin-point it if we saw the schematic but the search I did leads to a roof contractor. That Gibson amp was operating a field coil I think, so extra-current was needed to operate the field coil spkr. The field coil doubles as a filter choke.

[goldstache]

Exactly!!!
I’ve built a couple eh-150’s without the field coil. Whenever I get one in original, I gotta say, there’s some magic to the system with it there. Resistive or choke alternatives getcha there. But there’s a sweeter sustain with the old field coil!!


I didn’t draw it out, but in essence it was v1a 12ax7 gainstage (no cathode decoupler cap) into v1b cathodyne right to the cathode biased 6v6’s.
Had enough heater current for another dual triode (6112 submini in this case, they didn’t want holes drilled).
Added volume/tone and it sounds great. The input was for cartridge and had a 30ohm grid leak off that input.
So I did 5e3-ish v1a into slightly cold clipped v1b to existing amp circuitry.
Worked a charm.
Thanks again for the perspectives.

[tubeswell]

The power transformer HT center tap is floated from chassis ground with a big ol’ 330ohm resistor.  The amp is full wave rectified by a 5y3.

Its one way of providing Limiting Resistance to a High Tension winding to help limit the stress that peak ripple current puts on a tube rectifier to help the tube last longer. The HT winding does have its own internal limiting resistance, but sometimes this may not be 'enough' and so some peeps add additional limiting resistance. Sometimes you find one resistor each in series between each end of the HT winding and each respective tube rectifier plate. Other times it can be one resistor in series with the HT winding's center tap. Why don't you see this on all tube rectified geetar amps? Its a bit precious. Tubes can take more of a beating than you think. The stress of the tube also depends on the reservoir capacitance. If you have high reservoir capacitance, it might be a good idea to think about adding extra limiting resistance. But, a better way of protecting your amp from a tube rectifier failing short is to use SS diodes in series with the HT winding ends and the rectifier tube plates. YMMV