Alamo fiesta tremolo speed needs a tweak

[Mr Johnny Birchwood]

Hey guys. I modded the hell out of a '65 fiesta for a buddy. Not gonna go into detail except to say that its pretty brutal with 240vdc b+ and a 6aq5 opt tube, a 12au6(k biased instead of the stock grid leak) front end and a triode strapped 6ba6 driver tube in the socket where the 35w4 used to be(its now full-wave diode rectified.) I recapped the tremolo section with spec parts, and it works. The range of the speed pot goes from 'way too slow' to 'almost fast enough.' Can I change the range of adjustment by simply adding dc resistance either in series(net increase) or parallel(net decrease) with the speed pot? Also, the intensity has a good range of adjustment when the volume of the amp is low and the guitar is not being played hard, but when volume or attack is increased, the tremolo gets 'mushy' and barely audible. At max volume the tremolo intensity control is basically a sag knob. Neat effect, but not what I'm goin' for. Is it possible to make the tremolo intensity keep up with the volume/attack increase? The tremolo output is tied into G1 of the 6aq5, and I was thinking of maybe putting it into the grid of the 6ba6 drive tube or the cathode of the drive 6ba6 tube or output 6aq5 tube. The schematic I am posting is what the stock circuit was and does not show any of the mods I have done. I include it because I did not change anything in the 12av6 tremolo circuit. It was a pain in the arse to wire two different fil voltages(thank God for $3 recycled wall warts) but the result is so far pretty good.

[Jazz P Bass]

Alright. I'll bite.
Where is the 'Intensity' control?
Is it the 10K in the cathode circuit?(misdrawn perhaps)

[Mr Johnny Birchwood]

Intensity control is a 1M pot in series with 2.2M dcr between the .05 cap from the plate of the 12av6 and the 2.2M. The schematic was drawn from a 1962 model and this one is a '66. Different stock output tube, but besides the intensity control the circuit was basically the same. The other difference to note is that the resistors to ground in the tremolo network 220k and 470k are both 1M.

I don't know if the schematic was misdrawn, that weird little 10k squiggle thing from the cathode of the 12av6 caused some confusion for me. In the actual circuit there is simply a 10K resistor bypassed with a 10uf cap.

[Mr Johnny Birchwood]

I should further add that the 6aq5 is idling with 10.3vk across a 22uf bypassed 250ohm rk for 42ma idle current. Dropping 230v p - k for 9.6w idle. The triode-wired 6BA6 drive tube is idling with 5.03vk across un-bypassed 750ohm rk for 7ma idle current. Dropping 202b+ - 124vp across 10k rp. The front end 12au6 is idling at .504vk across a 33uf bypassed 2.2k rk. Dropping 202b+ - 87vp across 470k rp. G2 is sitting at 19vdc and g3 is tied to ground. I couldn't tell you the specifics of where the tremolo driver 12av6 is sitting without taking the readings. My other thought was to dump the tremolo return after the 2.2M into the grid or cathode of the front end 12au6. Thought maybe I would get a little guidance before I just plow ahead and re-arrange everything. I like the sound of the amp the way it is. Added a 1963 heppner AlNiCo 8incher in series with the stock ceramic CTS 8" for a total 16 ohm load. Got a good range of tones from 'crunchy' to 'frighten the devil' though the volume knob above 3 is basically a saturation knob. Great harmonics. Exactly what the guy was looking for but the tremolo effect could be better IMHO.

[Jazz P Bass]

That LF oscillator circuit as drawn is strange.
I thought the resistors to ground should be the same value.
470K sounds 'normal'.
What's up with the 1 meg?

[Mr Johnny Birchwood]

That's what was in there when I tore into it. They appear to be original. Brn-Blk-Grn is 1M. Tremolo didn't function when this thing showed up. I just recapped it with 50 cent metal film caps and it worked. Should I start by changing the value of those resistors? Let's call the(as drawn) 220k TR1 and the 470K TR2, with the 220K in series with the 500k speed pot TR3. What values do you suggest I swap in?

[Mr Johnny Birchwood]

One more clarification: The intensity pot is not a vr in series w 2.2M it is wired like a volume pot, one leg to ground.

[Jazz P Bass]

That's what was in there when I tore into it. They appear to be original. Brn-Blk-Grn is 1M. Tremolo didn't function when this thing showed up. I just recapped it with 50 cent metal film caps and it worked. Should I start by changing the value of those resistors? Let's call the(as drawn) 220k TR1 and the 470K TR2, with the 220K in series with the 500k speed pot TR3. What values do you suggest I swap in?

I would install what is on the schematic.

[Enzo]

There is a feedback/phase-shift loop from plate to grid through the three caps, that is why it oscillates. The caps and the resistors to ground make RC timing circuits. So the larger the caps, the slower the trem, and vice versa. Likewise the resistors - but opposite - the larger the resistances the slower the trem. SO 1megs instead of the 220k and 470 will be slower.

I am used to finding trems too fast - even the slowest setting is two or three times the minimum speed I want. I want the slow end to be a slow sexy throb.

[Mr Johnny Birchwood]

There is a feedback/phase-shift loop from plate to grid through the three caps, that is why it oscillates. The caps and the resistors to ground make RC timing circuits. So the larger the caps, the slower the trem, and vice versa. Likewise the resistors - but opposite - the larger the resistances the slower the trem. SO 1megs instead of the 220k and 470 will be slower.

I am used to finding trems too fast - even the slowest setting is two or three times the minimum speed I want. I want the slow end to be a slow sexy throb.

The low setting would be just dandy if it had more range of adjustment. Is there an easy way to increase the range? Could it be as simple as dropping the value of the 220k in series with the speed pot? Or maybe dropping the other resistors to 470k and using a 100k in series with a 1M speed pot.

As far as the intensity problem with higher drive, I'm just gonna disconnect and try different connection points with a clip lead to see if I get any improvement. I could live with the speed range if the intensity control were more functional.

[Jazz P Bass]

As far as the speed variability, how about a different pot?
Is the current one log or linear?
And the Intensity: certain tremolo circuits do not do well at full volume.
There is nowhere to tremolo at if the output section is slammed.

[Mr Johnny Birchwood]

That's why I was thinking of dumping the tremolo into the front end. The power tube is being pushed pretty hard to get the overdriven tones that the owner wants from this thing. It should be easier to turn the front-end 12av6 on and off with the LF-oscillator. Not sure if it is a log pot but I've got a few 1M sitting around. I think the 470k-470k-1M speed pot + 100k in series ought to get me closer to where I want it to be. It's not crucial, the guy that found this thing in a barn plays in a hardcore/metal type band and he just wants an apartment practice amp that will give him massive overdrive at manageable volume. That's exactly what this amp does now. He doesn't care all that much about the tremolo but I just want to make it better for my own sake: So I can increase my abilities and my understanding for future projects. I guess I'm just gonna have to experiment with it a bit..

[Enzo]

When it works, stop fixing it.

Really. YOu will have plenty of opportunities to learn and hone your skills. Thinking of re-engineering this little amp wouldn;t be my preferred method. If it does what the customer wants, move on.

Imagine this scene...
"Yeah, Hi, how are you? Listen, Um, I was working on your amp and I got it all tuned up, SOunded GREAT by the way, really cool amp, and um I was experimenting with the tremolo and... What? Oh I know you don't care about the tremolo, but I just wanted to see if I could get it to work different, you know, and I, well my probe slipped and I shorted out the high voltage, and well, the rectifier tube shorted out from that. Oh, no, rectifier tubes are not all that expensive, but it killed the power transformer when it happened. What say? Um yeah, I had it working GREAT, but well, I...."

Could it be as simple as dropping the value of the 220k in series with the speed pot?

DOn;t ask us, find out. YOu have a soldering iron and some parts. You want to know if lowering that resistor would speed up the trem? Put a clip wire across it, or clip a parallel resistor there. Zero ohms is about as reduced as it can get, or a parallel will lower the effective resistance. That will tell you if you are going the right way. And you could probably do it faster than you can post the question.

[Mr Johnny Birchwood]

Right as usual, Enzo. I have a couple of my own amps with non-functioning features that I can play around with.

[Mr Johnny Birchwood]

Guy came to pick up the amp this morning. He was very pleased with the 'hell and broken glass' overdrive and harmonics. Told him the tremolo was only semi-functional and he said, "What's That?"