grid leak biased... push-pull output stage?!

[mxrshiver]

where's the bias?! this is the earliest schematic for a commercial guitar amplifier i can find, the Rickenbacher 'Lunch Pail' amp from 1934, and i'll be damned if these push-pull type 47's aren't grid leak biased... what on earth is going on here how could that even work for output tubes

[HotBluePlates]

... push-pull type 47's aren't grid leak biased...

They are likely not grid-leak biased because the grid resistance is too low for that.  In preamp tubes we would expect 5-10MΩ, not 220kΩ.

where's the bias?! ...

See the star at the filament winding center-tap, talking about "notes in manual"?  That is probably where the cathode bias resistor was placed, between filament and ground.  Directly-heated tubes like these 47s would often do that.

Meanwhile, the 53 is indirectly-heated (it has a heater and separate cathode), and it uses that 860Ω for biasing both triodes.

[mxrshiver]

what a critical value to leave off a schematic! thanks, can ya tell i'm new to directly heated tubes? wanted to clone as basic a guitar amp as possible, to vicariously experience the earliest days of developing what a guitar amp even was, 'firsthand', problems and all, and understand changes that were made on an incremental level.

so, looks like the bias resistor connected the heater center tap to the ground for the rest of the circuit? the 47 datasheet suggests a 450R cathode resistor, so i'll start with a 5W 910R for the two 47's, see how it looks. i've heard hum can be an issue with these - would it be just as fine to tape off the center tap, bridge the heater winding with a 100R WW pot, then send the wiper through a 820R 5W to ground?

[mxrshiver]

whoops, make that a 225R 5W haha

[HotBluePlates]

... wanted to clone as basic a guitar amp as possible ...

The obsolete tubes and field coil speaker might stop you in your tracks on this project.

You might also look to see this amp will be clean & wimpy.  It is just a phase inverter and a pair of push-pull output tubes, with mostly no preamp.  The Type 47 is kind like a 6V6 with lower max ratings & needing more drive signal.  The Type 53 is a power tube itself, but is being used as a voltage amplifier with. gain kinda like a 12AY7 (but much lower output impedance).

I think you could build the largely the same amp copying only the power supply, power section and phase inverter of an early Fender Tremolux.  And I think you'll be underwhelmed and miss the extra gain and convenience of the Tremolux' Volume and Tone controls (plus additional preamp stage).

[shooter]

if you own it and wanna get it running 
if you're thinking about building it, take HBP's advice, do something else.

[mxrshiver]

http://prewaramps.org/audioclips.htm

check the clip - not so clean after all, and with lower output pickups modeled after those from 1952! definitely a bit of a one trick pony, and blocking distortion is gonna be a fundamental part of the distortion i get, i'm sure, but i think it sounds very pretty and has expressive capability in the distortion. i figure the higher voltage output from modern pickups, along with the fact that the 53 is a power tube with plenty of current, is enough to push the output into some genuine overdrive. got plenty of 47's and a couple 53's to play with, though i will have to sub a choke and power resistor for the field coil speaker - i actually have a couple from old radios, but they're made for 71A and 45 outputs and likely don't have enough power handling. but i'm very stoked on building an mostly authentic amp that uses the old 2 digit tubes, an imperfect, 'proto-amp' with an open layout and extra terminals so i can mess with it. at the very least, it'll be a neat lil old school power amp to run other preamps into. definitely plan on including volume and tone, as i'd have exactly the same amp when they're dimed.

so, is it alright to tape off the filament winding CT, bridge the 2 with a 100R humdinger pot, and follow with a 125R cathode resistor grounded to the rest of the circuit? figuring the cathode would see 225R total that way, but not sure if it'd see the full humdinger resistance or just one half.

i'm also very curious about the B+ here. schematic shows a 890V CT winding into a type 80, which should make somewhere in the neighborhood of 460VDC rectified. but this is only run through a choke and the field coil before it hits the screens, and the 47's screens are only rated at 250V! roughly estimating the total load current somewhere around 100mA, to drop 210V across the choke and field coil would require 2.1Kohms, and dissipate 21W! guessing that was pretty much all in the field coil, could they really dissipate that much heat? since i'm not using the field coil, seems i could save myself significant size, weight and expense on the power transformer getting one that's more like 550VCT and just dropping it across one fat choke, maybe 20-40H to combine the former choke and field coil inductances, and 400-500DCR to land the voltage hitting the 47's just below 250V. seems to me it would sound and respond about the same?

[PRR]

> the fact that the 53 is a power tube with plenty of current, is enough to push the output into some genuine overdrive

Drive a 900 horsepower engine fed through a wee-thin fuel line. You don't get 900HP.

It's got 220k in series with the power rail. You can't possibly get 1mA toward the power tube grids, and that only for the few milliSeconds before the grid-caps charge up.

The only excuse for the 6A6/'53 is that it went out of style very fast so there were vast surpluses for cheap.

[HotBluePlates]

http://prewaramps.org/audioclips.htm

check the clip - not so clean after all, and with lower output pickups modeled after those from 1952! ...

Those are P-90s, so pretty hot (even if exactly matching vintage pickups).

Note also the amp does not have a Volume control.  Unless the volume was turned way down at the guitar, what you're hearing is that Rickenbacher dimed.

I used to own a 1955 Tremolux... You'd get the same response at about 3 (out of 12) on the Tremolux.