vibrato speed not slow enuff

[Alan Lenhoff]

Slowing down the Tremolo on a Twin Reverb

The Fender tremolo oscillators all work the same. There are three caps in series from plate to grid on the trem oscillator triode. From the plate they are 0.02uf, 0.01uf, and 0.01uf. I find it easiest to simply raise the two 0.01uf caps to 0.02uf. You can either replace then, or jut put a second 0.01uf in parallel with the existing ones.

I know this is an old thread, but this seems to be the solution to the too-fast tremolo on my '77 Twin Reverb. These are ceramic disk caps, right? And what voltage rating would I need to buy?

Thanks,

Alan

[jmaf]

I know this is an old thread, but this seems to be the solution to the too-fast tremolo on my '77 Twin Reverb. These are ceramic disk caps, right? And what voltage rating would I need to buy?

Thanks,

Alan

They can be poliester or polietilene as well if you can't find higher voltage ceramic disks out there. Must be rated 600V or more.

[Enzo]

I think ceramics work well, and they are cheap. This is only an oscillator, not the signal path, so there is no "tone" involved.

They need to be of enough voltage to handle the circuit.

[Alan Lenhoff]

Thank you both for the info. I see 1KV ceramics readily available, so I'll go with those.

Alan

[Alan Lenhoff]

While I am experienced with replacing components in relatively low voltage solid state keyboard instruments, I have not worked on tube devices with high voltages before. On these caps in the tremolo circuit, do I need to be concerned about high voltages being present? (The amp is a 100 watt, 1977 Twin Reverb.)

I realize that this may be a trigger for some of you to tell me I ought to take the amp to a tech. (And if so, I can stand hearing the truth. Better safe than sorry.) But maybe this isn't an issue in this part of the circuit?

Any advice?

Thanks!

[g1]

Discharge all the high voltage caps before doing any work in there. Double check that they are discharged with your voltmeter.

[bob p]

Wow, Necrothread. I was surprised to see SGM offering advice. For a minute there, I thought he was back!

Enzo gives good advice. But there are still some variations in the Trem/vibrato circuits that would render invalid the rule that "they are all the same."

The Tweed and Brown/Blonde circuits are notable exceptions. The Tweed circuits typically used bias-vary tremolo while some of the Brown-era circuits relied on a very complex quadrature-voltage phase-shifting vibrato that often gets called "tremolo." The statement that all Fender circuits are the same is largely true in the 763-type amps of the BF/SF era, where the circuits use the optoisolator bug. That said, there are still some smaller SF/BF circuits that still used the power tube bias method, rather than the opto bug, so those would be notable exceptions to the rule.

If you're looking at an AB763 BF Twin or it's SF descendants, the circuits are going to be the same basic opto bug design.

[bob p]

Wow, Necrothread. I was surprised to see SGM offering advice. For a minute there, I thought he was back!

Enzo gives good advice. But there are still some variations in the Trem/vibrato circuits that would render invalid the rule that "they are all the same."

The Tweed and Brown/Blonde circuits are notable exceptions. The Tweed circuits typically used bias-vary tremolo while some of the Brown-era circuits relied on a very complex quadrature-voltage phase-shifting vibrato that often gets called "tremolo." The statement that all Fender circuits are the same is largely true in the 763-type amps of the BF/SF era, where the circuits use the optoisolator bug. That said, there are still some smaller SF/BF circuits that still used the power tube bias method, rather than the opto bug, so those would be notable exceptions to the rule.

If you're looking at an AB763 BF Twin or it's SF descendants, the circuits are going to be the same basic opto bug design. But I agree -- it always helps to post the schematic for the amp that you're asking about.

[g1]

I should clarify that where I said high voltage caps I meant the main filter caps in the power supply circuit. These are the ones that can hold a charge when the unit is turned off.

[Alan Lenhoff]

Thanks everyone! The job is done, and the slower tremolo on my Twin Reverb sounds fabulous with my Fender Rhodes piano. (But you can still turn up the speed so it's fast enough to sound like a blur. It's a perfect range.)

I worked VERY carefully on this because I'm not experienced in working on tube amps. I was going to buy -- or make -- a capacitor discharge tool. But then I saw a video online that indicated that my Silverface Twin has a bleeder circuit. So, I just verified with my meter that it worked properly: I turned on the amp long enough for it to fully warm up and be playable, noting that the positive ends of the filter caps were measuring about 370 volts. I switched the amp off (without touching the standby switch) and watched the voltage quickly drop to 6-7 volts. And after a minute or so, the voltage was less than one volt. That sounded safe enough to me. Added two 0.01uf 1KV caps in parallel with the existing 0.01 uf caps. (I did this rather than replacing the existing caps with higher value ones because I wanted to leave it evident that a mod had been done, in case I ever sell it.)

If I've given any bad -- or unsafe -- advice here, I hope someone will correct me. (And I'll edit my post to make sure I don't lead anyone into electrocution.)

Anyway, thanks again!

Alan

[g1]

Well done and well thought out. That's how I discharge, use the amps own bleeders if possible (via standby switch in play position), then check and further discharge if necessary.
Also like your thinking on the parallel caps to make the mod stand out and be easily returned to stock.

[bob p]

Thanks everyone! The job is done, and the slower tremolo on my Twin Reverb sounds fabulous with my Fender Rhodes piano.

I'm guessing that you've got the 4-legged Stage model piano, right? Neoprene or felt tipped?

I think those sound absolutely fabulous through a Twin Reverb.

[Alan Lenhoff]

Well done and well thought out. That's how I discharge.

I am hardly a brilliant electronics person. But I take pride in being smart enough to know what I DON'T know. I try to overcome that by listening well, and working carefully and cautiously. (Come to think of that, I guess that's how you learn!)

By the way, I just recently discovered this group, and I'm finding a lot of excellent and very helpful people here.

Alan

[Alan Lenhoff]

I'm guessing that you've got the 4-legged Stage model piano, right? Neoprene or felt tipped?

I think those sound absolutely fabulous through a Twin Reverb.

Yes, I have a 1974 Stage with Neoprene tips that sounds great through my Twin. The Rhodes pianos varied considerably by year in tone, action and craftsmanship. '74 was an excellent year. You can see my baby here:

https://vintagerockkeyboards.wordpress.com/

Alan

[bob p]

Nice. I remember when Rhodes came out the neoprene tipped Stage pianos. I was playing bass back then and our piano guy ended up buying the smaller 73 and playing it through a Super Twin Reverb. There was something special about the way he was able to get that Super Twin dialed in -- he used a combination of the masters and the EQ to add some subtle "bite" to the Rhodes tone, keeping it clear but on-edge no matter what sort of volume level we played at. He had to trade away the tremolo feature of the standard TR to get that, so he bought an MXR phaser to fill that niche. Phasors were the big deal at the time -- and "Just the Way You Are" was a popular cover back in '77.

I see that you have a D6 Clavinet. I've always loved the sound of those.

And I see that you're using a 490. I'm wondering if you've given any thought to using the ST for your piano collection, stretch tuning and all.