Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DummyLoad on June 14, 2020, 12:52:10 am
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digging my dykes into this old lafayette 35W PA amp. tube compliment is: 12AX7 (1st stage MIC1 & MIC2 preamps), 6AV6 for the summing, radio & phono amp, a second 12AX7 for final gain stage that drives a self-biased cathodyne in turn driving a pair of 6L6GC in AB, fixed bias.
the 6L6GC are lafayette branded GE OEM, the 12AX7 as MIJ and 6AV6 is OEM GE. the 6L6GC are in like new condition. it does not look like this amp had much use, however was not stored well for a while as it has rat urine stains and the ensuing corrosion from that on top of the electronics chassis.
other than creature stains & corrosion, it's in decent shape for its age - 1964-ish. interesting that it has solid state FWB HT supply. come to me with a fried 560R 2W resistor in the plate supply to screen supply RC network: someone didn't use the slide-rule proper, or they figured 2W will get really hot but last just long enough. otherwise this thing is unmolested. e-caps were OK, but getting tired. this one has a james stack sandwiched between summing amp and final amp. the james stack employs a hybrid module for the RC networks. pots are twist tab mount - haven't seen that in long time.
mods:
upped the PS plate and screen filter cap values to 47uF IC brand. replaced burned and cracked 560R 2W with 470R 6.5W pulse rated WW. 4.7uF 1st stage amp PS filter upped to 10uF.
bias supply 10uF 100V caps upgraded to 22uF 100V nichicon.
grid leak bias for V1a & V1b, mine had neither .02uF grid coupling caps (EDIT: i am a dumb-ass! of course they are! won't work without them!) as shown in the schematic. adding 3.3K cathode resistors un-bypassed for both V1 triodes.
cut the fader, radio, phono bits out and rewired 2M fader pot as MV: yes, it's linear taper - who cares, it's there! :p and we ignore the 50% tap.
clipped out the AC auxiliary receptacle. adding a 3 wire grounded power cord. added a 1/4" pone jack for speaker connection.
am happy: rosin is smoking once again, drills are drilling, and dremels are cutting. :icon_biggrin:
--pete
EDIT: see in-line!
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schema for most of the mods. the wiring changes mostly.
--pete
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> or they figured 2W will get really hot but last just long enough
I don't know a 6L6GC amp which would put more than a part-Watt in that resistor. It met a bad tube.
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> or they figured 2W will get really hot but last just long enough
I don't know a 6L6GC amp which would put more than a part-Watt in that resistor. It met a bad tube.
or a shorted/dying cap.
--pete
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Perhaps they matched the 100k 1W 5% resistors in the PI sufficiently well to add that 2k2 to tweak the balance 'just right'!
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am happy: rosin is smoking once again, drills are drilling, and dremels are cutting. :icon_biggrin:
:thumbsup:
My furnace kicked on, the neighbors wood heat is smoking, powers out down the street, I'm going back to bed :icon_biggrin:
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am happy: rosin is smoking once again, drills are drilling, and dremels are cutting. :icon_biggrin:
:thumbsup:
My furnace kicked on, the neighbors wood heat is smoking, powers out down the street, I'm going back to bed :icon_biggrin:
you'll get all the sleep you need when you're dead. build something before the full-on apocalypse hits. :icon_biggrin:
--pete
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:l2:
It hit, now no power here, I mowed! then fired up the solar truck.....just in case :icon_biggrin:
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Perhaps they matched the 100k 1W 5% resistors in the PI sufficiently well to add that 2k2 to tweak the balance 'just right'!
i suppose that's to balance the whole stage - compensates for 2.2K cathode resistor. IMO, a waste of a part.
--pete
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It's likely they did a sample check on each batch of 100k 1W 5% resistors and found the batch had pretty good relative tolerance (eg. within 1% of each other, although not necessarily within 1% of 100k).
Given they have included feedback with some additional high frequency compensation, and adjusted the bias point of the feedback stage, and have typical domestic speaker impedance options, plus other nods to hum, this seems to be more a moderate hi-fi amp with PA capability tacked on, so I reckon they would have taken some care to ensure good symmetry.
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ok, now this is a fun amp. converted 1st gain stages to self-bias. had too much LF boom on the neck PU + was too easy to overdrive. i wired in mismatched bypass and bias bypass caps for each channel since inputs wired with both channels in tandem. attached schema of as built now with telemetry.
--pete
https://www.dropbox.com/sh/buzda189emkcdym/AACDiff5lWXIfhFfdWhhEo0Aa?dl=0 (https://www.dropbox.com/sh/buzda189emkcdym/AACDiff5lWXIfhFfdWhhEo0Aa?dl=0)
EDIT: add link to project photos.
EDIT: fixed schematic - .sch and .pdf formats updated.
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Do the PI and output stages need grid-stoppers given you may thrash the pants off it, and the 100nF coupling caps may make any blocking distortion a bit gross.
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Do the PI and output stages need grid-stoppers given you may thrash the pants off it, and the 100nF coupling caps may make any blocking distortion a bit gross.
not at all. quite pleasant to play actually - hifi-ish tone. i was going to add them, but that layout doesn't make it easy. simply converting from grid leak bias to autobias for the 1st stages was enough of nightmare with all that packed in mess.
the smaller OT takes out a lot of the boom, along with the TS. i like the bridge pickup of my strat with the bass at 1-2 and the trable around 7-8. kicks @$$ loud and takes some drive to get it breakup. doesn't have much feedback, just goes from loud clean to a louder harmonically rich crunch. humbuckers could probably push it over the edge.
--pete
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The re-draw of the two 68k 1W makes it look like they go to the grid, a mis-impression.
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The re-draw of the two 68k 1W makes it look like they go to the grid, a mis-impression.
no dot - no connection! ;-) i'll fix by moving the upper R below the grid.
--pete
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fwiw;
I use the circle icon on left, pick arc on top, size radius to .08, makes a nice "wire" jump