Possible mod for increasing tremolo strength in Silvertone amp - good idea?

[lassoharp]

I've got a Silvertone 1474 that had really weak tremolo even after all the osc circuit caps had been replaced, resistors checked, tubes swapped etc. 

The oscillator was putting out voltage, it just apparently wasn't enough to drive the 6L6s to the same Intensity level of the 1482/72, or the 1474s I've seen on you tube so I'm assuming the stock amp was intended to have a stronger tremolo than I was hearing before attempting the mods.

The 6L6 bias and plate voltages look well within the schematic values -  25.5 vs 27 (stock)  344-346 vs 340 (stock)

The 1474 uses the same tremolo circuit as the 1482/72, which used a pair of 6V6.  The only difference is the added 220K resistor from the Intensity wiper to ground. 

I removed the 220K and subbed in resistors and noted the effect.  680K-1M sounded about right.  The oscillating voltage at the intensity pot wiper cranked wide open was peaking at about 15V.  And, the 6L6s showed no signs of red plating.

Anyone see any potential problems with raising the 220K resistor?  Or see a better way to increase the Intensity strength?

 

[PRR]

You could reduce the 330K to maybe 100K.

When you go "too far" the trem will go splat splatt. Backing-off will fix it. Adjust the "330K" so "10" is just shy of splat.

I dislike taking the 220K much higher because, even self-bias, we are pushing the maximum grid resistance. 500K for 6L6 in self-bias. 220K obvious, so we can have another 280K each. If this leg is combined it "should" be 140K max. 220K||1Meg is 180K, a shade high already. We know most g-amps violate this spec, but why push it?

 

[CJ]

what kind of caps do they use in the trem circuit?  if they leak from being old then you might lose some wiggle,

yanking the pwr tubes around is always tough, maybe try routing the vibrato to one of those 2.2K cathode resistors like the vibro champ, might have to divide down the signal, might be too noisy,


OT:  look for arcing on the pwr tube sockets, these amps are notorious for having the sockets catch for after arcing,

check rivets, loose rivets in some places might mean a shaky ground,

check grid stoppers, if they are 68K consider 5 or 10 k unless you like the mississippi mud sound

check OPT's for  pri DCR,  if a large difference is noticed then you might need a rewind on one of both,

stereo output transformers could be made into wicked vibrato aka magnetone circuit,

 

[trobbins]

Just to clarify a few uncertainties:
- was the tremelo previously 'good'?
- is the circuit stock or modded?
- is the amp working fine for normal signal path to both speaker outputs - same expected loudness?
- do both output stages have balanced idle current in each side?

 

[lassoharp]

You could reduce the 330K to maybe 100K.

When you go "too far" the trem will go splat splatt. Backing-off will fix it. Adjust the "330K" so "10" is just shy of splat.

I dislike taking the 220K much higher because, even self-bias, we are pushing the maximum grid resistance. 500K for 6L6 in self-bias. 220K obvious, so we can have another 280K each. If this leg is combined it "should" be 140K max. 220K||1Meg is 180K, a shade high already. We know most g-amps violate this spec, but why push it?

Thanks PRR,  I totally forgot about the max grid resistance spec.

Lowering the 330K to 100K worked well. 

what kind of caps do they use in the trem circuit?  if they leak from being old then you might lose some wiggle,

I think the originals were Sangamo molded types.  Somebody had went in before me and put brand new modern film caps.

check OPT's for  pri DCR,  if a large difference is noticed then you might need a rewind on one of both,

Knock on wood. LoL  They measure 255/228 and 252/224 and are a little undersized.  If one goes I'll send it to you.  Amp actually belongs to a friend of mine and he wants to keep it as original as possible.

stereo output transformers could be made into wicked vibrato aka magnetone circuit,

Yep      Wish it were mine to do that with. 

Just to clarify a few uncertainties:
- was the tremelo previously 'good'?

When the amp was brought to me the tremolo wouldn't work at all.  I replaced the coupling caps from the PI to the power tubes which were leaking badly and this brought the tremolo back to life a little, but it was still very weak.

is the circuit stock or modded?

As best I can tell it's stock.  All the tremolo circuit caps had been replaced by a previous tech, but the values were stock.

- is the amp working fine for normal signal path to both speaker outputs - same expected loudness?

Yes

do both output stages have balanced idle current in each side?

I'd say so.  The plate voltages varied from 344-346 which is pretty good.  The previous owner had installed what looks  like a matched quad of Sovtek 5881