Variable Tremolo Intensity for Gibson GA-5T Skylark?

[rock soderstrom]

Hi guys, I'm planning a new project right now. A Gibson GA5T Skylark clone with a few upgrades.

There are incredibly many, partly completely different, amps under the designation GA5T. I'm talking about the following variant. (12AX7/6BM8/5Y3GT).

My question is: How can I vary the intensity of the super sounding tremolo effect?

My idea is to replace the lower part of the V2B grid resistor with a 250K poti. Will this work? Lin or Log?

Cheers

 

[Tubetec]

I'd highly reccomend Jack Darr's guitar amplifier handbook , first edition .
Its all tube stuff and theres a great chapter on vibe/trem with examples from Fender,Gibson and other USA manufacturers.
Someone gifted me a copy they never returned to the library , just as well because it more than likely would have been dumped at this stage had it been .

Chapter 2 special signal circuits ,
https://ozvalveamps.org/jackdarrhandbook.htm

 

[rock soderstrom]

Thanks for the nice book link!

Here again my question. Will it work this way?

 

[CJ]

that should work,
ng,
if not you could inject the second stage cathode like the fender vibro champ,

interesting to see if the speed changes while you adjust that pot, or if the sound changes or if the circuit quits oscillating,

 

[rock soderstrom]

that should work,
ng,
if not you could inject the second stage cathode like the fender vibro champ,

interesting to see if the speed changes while you adjust that pot, or if the sound changes or if the circuit quits oscillating,

Thanks, I will test it this weekend. The tremolo of this Skylark is unbelievable good, even better than the vibro champ!

 

[Tubetec]

The first simplified example from guitar amp handbook is more or less the same with minor details left out .
I'll dig out my original 1965 edition later and see if theres anything interesting in there .
It may be worth paying extra attention to filtering and smoothing of the HT in a single ended amp like this , trems can end up modulating mains hum and superimposing it onto your guitar signal , it has an effect on the sound/tone of the amp due to intermodulation and probably transformer saturation to some degree.

Edit
The OZ valve amp page has more or less everything from the first edition , one other interesting circuit to look at is the LDR/lamp trem on page 29 .

 

[rock soderstrom]

The first simplified example from guitar amp handbook is more or less the same with minor details left out .
I'll dig out my original 1965 edition later and see if theres anything interesting in there .
It may be worth paying extra attention to filtering and smoothing of the HT in a single ended amp like this , trems can end up modulating mains hum and superimposing it onto your guitar signal , it has an effect on the sound/tone of the amp due to intermodulation and probably transformer saturation to some degree.

I have seen it, this solution is slightly different, but it is the same idea. I will test it all this weekend. The same goes for the PSU. I will build the Skylark stock first to see how the whole thing sounds and if the noise is annoying.

In my experience, you have to be careful with additional filtering in the PSU with such amps, if you install too large caps and chokes, the whole thing becomes too "hard" and the amp loses some of its charm. It is a fine line.

We'll see, I already have some good mod ideas....

BTW, does anyone see a clever way to loop in a spring reverb? Right now I would just put the reverb tank in front of it, like an external device, e.g a 6G15.

The amp definitely needs reverb for my style of music, Gibson never did that AFAIK in the GA5 series, which probably had financial reasons as the GA5 was an entry level model. Especially such small boxes benefit very much from reverb.

Edit: Here is the schematic of the Gibson GA-1RT-1. Its a very similiar amp with reverb, but you need a second driver amp. Strange!

 

[Tubetec]

Mains noise modulation adds extra fuzzley wuzzley to the amps distortion for sure ,
crystal clear mode might be nice to though .

What about a low Z tank driven in parallel with the speaker , get as high an impedence as you can in the recovery coil and feed it in via a 470k mixing resistor to the second stage grid , vary the drive level to the tank for swell control .Unless you need a sound totally washed out with reverb it should work fine .

 

[rock soderstrom]

What about a low Z tank driven in parallel with the speaker , get as high an impedence as you can in the recovery coil and feed it in via a 470k mixing resistor to the second stage grid , vary the drive level to the tank for swell control .Unless you need a sound totally washed out with reverb it should work fine .

Sounds good, thanks for advice.

 

[CJ]

Yeah that is really weird clipping alligator clips to a speaker, I had to meditate for three hours to get that concept erased from my memory.

The best reverb I have ever heard split the input signal from the jack into a 12ax7 driver, instead of pulling it from the first or second stage amp,

 

[radardoug]

What about a low Z tank driven in parallel with the speaker , get as high an impedence as you can in the recovery coil and feed it in via a 470k mixing resistor to the second stage grid , vary the drive level to the tank for swell control .Unless you need a sound totally washed out with reverb it should work fine .
Thats going to feed back when you wind up the reverb. Not a good idea.

 

[alphasnk]

Rock, apparently, Uncle Doug did something to that tremolo circuit to control the intensity, i havent watched it yet:

 

[abbey road d enfer]

Thanks for the nice book link!

Here again my question. Will it work this way?

View attachment 97582

Only to an extent. At very low settings of the pot, it will load the oscillator, which will simply cease to oscillate. You need to reverse th econnections to the pot, i.e. wire the oscillator output to the top of the pot; ground the bottom and connect the wiper to the grid resistor. It will give you a better control of depth.

 

[Bo Deadly]

I'm not a tube guy so this is just a wild shot-in-the-dark but maybe replace the 1.5M in the oscillator filter with a pot and feed the grid of the oscillator tube from it's wiper.

 

[rock soderstrom]

Thanks for the suggestions! I'm just doing the PCB, I have no more Turrets in stock, so I have to do it this way. Blaphemy.

I will try all the ideas, I think the suggestion from tubetec's book or abbey's will be the best solution.

For the reverb I'll go with my first idea, I'll simply build a tube reverb tank in front of the amp in the same cabinet as an add on. I still have some current left on the heating coil and since the PSU already exists to a large extent, the effort is kept within limits. Surf on!

 

[abbey road d enfer]

I'm not a tube guy so this is just a wild shot-in-the-dark but maybe replace the 1.5M in the oscillator filter with a pot and feed the grid of the oscillator tube from it's wiper.

There's not enough signal there. Typcally the tremolo oscillator tube operates with about 30-50 gain, so the signal at the anode is about 30-50V and only 1V at the grid.
In order to produce significant gain modulation to a cathode-biased tube, it takes about 5-20V p-to-p superimposed to the grid.

 

[rock soderstrom]

This is partly an upcycling project. Here are some SE transformers from 3.5W to 10W. I'm curious to see how the old Grundig iron performs against the modern Hammond 125DSE.


Does anyone know the dimensions of the original GA5T output transformer? I bet it is rather small.

 

[abbey road d enfer]

For the reverb I'll go with my first idea, I'll simply build a tube reverb tank in front of the amp in the same cabinet as an add on.

It's a wise decision. The Gibson RT1 is a reverb add-on, which is to be grafted to and existing amp. In order to make it more appealing, they added guitar inputs.
Different choice than Fender, who opted to add the reverb at the input.
Many years ago, I toyed with the idea of picking up signal from the speaker to feed the tank and reinjecting at the input, as Tubetech suggested
It poses two problems:

• Unless there is a dedicated stage, mixing the reverb signal with the input impairs the guitar sound
• As hinted, there comes a time where nasty feedback happens

 

[Bo Deadly]

There's not enough signal there. Typcally the tremolo oscillator tube operates with about 30-50 gain, so the signal at the anode is about 30-50V and only 1V at the grid.
In order to produce significant gain modulation to a cathode-biased tube, it takes about 5-20V p-to-p superimposed to the grid.

Not sure I follow. With the wiper at max, it would be an equivalent circuit.

 

[Tubetec]

Ive had excellent results attenuating down the speaker output to the spring , the speaker remains connected and miked up as usual .
yes theres a chance of regeneration to the point of oscillation but its controlable , in something like a vintage Marshall with two channels you can easily add back in the reverb on the extra channel , I can tell you not only does it work, with things set up right it sounds great , the amount of re-gen varies depending on setting of the controls and helps thicken out the sound of the spring .

A high impedence mixing stage like in a Marshall ,yeah its got loses , didnt stop all the great sounding albums made through them though did it ? Fender Bassman has the same set up .

The old German radio sets have great transformers , Saba Grundig Telefunken ,well worth using for project stuff .