FrankenstAmp build

[Paul P]

I installed the second channel preamp and that went ok. The volume was
much lower coming out of this channel because of the voltage divider on
the output.

I then installed the tremolo board, turned the amp on and got...nothing.
Dead as a door nail. I got some oscillator activity in the mV range at the
power tube but no sound. The clean channel worked ok. Pulled out my
cheap oscilloscope that I'd never used before and traced the input signal
to the preamp's coupling cap which had a signal on one side and nothing on
the other. I desoldered one leg of the cap and it tested good. The output
side though had zero resistance to ground. Followed the wire to the tremolo
board and found it connected to a ground turret instead of the 220K input
resistor . Fixed that and got sound !



This tremolo circuit is pretty neat. You can hear different things happening
at different frequencies. For instance, sometimes you get a beat going in
the base while the treble is doing something else.

One thing, though, is that the volume is still quite a bit less for the tremolo
channel that it is for the clean channel. That voltage divider is only letting
through about 10% to the tremolo circuit. I hope I can up this a bit without
causing anything to overload in the tremolo. There is also a bit of fizz sometimes
in the upper frequencies but it may be because the preamp was a bit over
driven. The overall sound is a bit darker too, not as clear as the clean
channel. The .003 cap across the output might have something to do with
this. It's shown in some schematics and is missing in others.

I still have to play a bit with the speed and intensity controls which are using
rotary switches and I don't have suitable values for the resistors.

Oh, and I installed a second 6v6 in parallel with the first. I had to use two
coupling caps, one for each 6v6, because my individual bias supplies were
connected together when I used a "Y" from the single coupling cap. It's
louder, but I wouldn't say twice as loud. I'm still playing through a cheap
hi-fi speaker (85-90db ?) so I can't really evaluate the volume at this point.

Paul P

[Paul P]

I've added the reverb circuit so my amp is more or less complete :



I can still go to a push-pull output by adding the PI board, a tube, and
replacing the little output board but I won't do that for the time being.

The reverb sounds pretty good though I can overload the mixer triode
if I turn the "reverb" knob up too high. Also I've currently got only 376v
driving the reverb transformer compared to 410v in an AB763 Deluxe
Reverb. This may explain the fact that although I can get a nice amount
of reverb, I can't really get it to a point where there's too much.

I still have to play a bit with the bits and pieces connecting the different
circuits together. Since Fender never did put all of them together I'm
pretty much on my own. The amp still has no hum, thank God for that.

Paul P

[tubeswell]

I've added the reverb circuit so my amp is more or less complete :

Paul P

Hi Paul P - You got any sound samples of it? I am building something between a 5E9A, 5F11 and a 5G9 (I think?) and I am keen to hear trem samples.

Cheers

[Paul P]

You got any sound samples of it? I am building something between a 5E9A, 5F11
and a 5G9 (I think?) and I am keen to hear trem samples.

Hi tubeswell. I'll see what I can do. The sound won't be the greatest since
I'm currently playing through a cheap, inefficient hi-fi speaker (sealed 6" 2-way
acoustic suspension box) but maybe you could still get an idea of what the
tremolo sounds like.

And I'm not much of a guitarist at all, which won't help .

Paul P

[Sir Cuitous]

I agree - nice shop setup. My current project is a lot less complicated and I'm already bitching about the drilling and punching. I have a question, are you going to be able to flip a switch between the PP and SE OTs? It does not look that way from your photo.

[Paul P]

I have a question, are you going to be able to flip a switch between the PP
and SE OTs? It does not look that way from your photo.

No, I have to swap output transformers and the little output board. I haven't
thought too much about push-pull operation other than working out the board
layouts and space just to make sure I could go that route later on. Now that I
think about it though I guess I should at least make the phase inverter and
push-pull output boards while I'm still set up for that kind of stuff.

I understand that you can use a push-pull output transformer for SE output
but that it's not the greatest due to the lack of an air gap. Victoria amps
apparently have a transformer that gets around this problem. I wouldn't mind
knowing what the trick is.

Paul P

[Sir Cuitous]

Thanks for the reply. I have to say your design and workmanship are top notch. Thanks for taking the time to document your build. I too would be interested to know how Victoria does that, though I'm not sure I would ever design an amp that way.

[Paul P]

Here are some samples of the tremolo. Unfortunately I had to severely
reduce the bitrate so the quality has suffered, mainly the bass and some
richness of the detail. The recording was made by mic'ing my cheap
speaker into an Edirol R-1 using an EV/Telex Cobalt 11 microphone. I
played my daughter's MIM Fender Strat Jr. The clicking you can hear is
the rotary switches I'm using to select the speed and intensity.

I was a bit depressed to hear some hum in my headphones while setting
things up. I guess it had been masked by the lighting in my garage. It's
worse with the reverb turned all the way up. Something I'll have to look
into...

trem64_1.mp3

trem64_2.mp3

trem64_3.mp3

trem64_4.mp3

trem64_5.mp3

trem64_6.mp3

trem64_7.mp3

Message to tboy : if these take up too much space feel free to delete them.

Paul P

[tubeswell]

Here are some samples of the tremolo. Unfortunately I had to severely reduce the bitrate so the quality has suffered, mainly the bass and some richness of the detail. The recording was made by mic'ing my cheap speaker into an Edirol R-1 using an EV/Telex Cobalt 11 microphone. I played my daughter's MIM Fender Strat Jr. The clicking you can hear is the rotary switches I'm using to select the speed and intensity.

I was a bit depressed to hear some hum in my headphones while setting
things up. I guess it had been masked by the lighting in my garage. It's
worse with the reverb turned all the way up. Something I'll have to look
into...

Paul P

Thanks Paul P

I'm downloading them to iTunes to have a listen

Regarding the hum. Are you sure its not just your guitar? (Is there much/any hum without the guitar plugged in?) If not, then try the guitar shielding mod. I did this to my strat recently and it works wonders.

[Paul P]

I think the hum is from the reverb transformer being real close to the PT.
I haven't had a chance to try putting a shield between the two to see
if that makes a difference. The hum is not affected by the inputs. Even
with nothing connected and volumes at zero the hum behaves the same,
ie, it's a function of the reverb pot's rotation.

Paul P

[tubeswell]

I think the hum is from the reverb transformer being real close to the PT.
I haven't had a chance to try putting a shield between the two to see
if that makes a difference. The hum is not affected by the inputs. Even
with nothing connected and volumes at zero the hum behaves the same,
ie, it's a function of the reverb pot's rotation.

Paul P

Maybe you could put a bit of aluminium sheet between the two trannies, or better still build a little aluminim doggie-box to go over the reverb tranny. Leave some venting in the top of it.

Or another idea for a simple quickfire troubleshooting experiment for unwanted coupling (I came up with this one time when modding my C30) - get a small block of polystyrene foam and wrap it in Aluminium kitchen foil and jam it bewteen the two trannies (making sure you don't short anything). If the hum reduces this way, you'll know you're on to something - then you can go to the effort of building the doggie-box.

BTW I like the trem samples. Did you use a trem cicuit in the pre-amp or in the power amp?

[Paul P]

Did you use a trem cicuit in the pre-amp or in the power amp?

I guess this explains why you asked about my circuit. See message
#30, a few messages back in this thread :

http://music-electronics-forum.com/s...0320#post50320

With respect to my reverb transformer, wouldn't an iron shield be better
since this is a magnetic problem ? I'll try both to answer the question.
Maybe later this evening.

Paul P

[tubeswell]

With respect to my reverb transformer, wouldn't an iron shield be better
since this is a magnetic problem ? I'll try both to answer the question.
Maybe later this evening.

Paul P

Aluminium is non-magnetic and therefore provides excellent electrical shielding. If the problem is engendered by electro-magnetic interference, then an Aluminium barrier should stop it.

[Sir Cuitous]

EMI can be shielded by any conductive metal. Iron is therefore not the only material that works. The greater the conductivity the better the shielding property. In terms of shielding, conductivity not magnetism is the key factor. Magnetism can create an electrical current, but any conductive material can shield that. Aluminum is highly conductive and will work very well as a shield. Also, make sure the shield is connected to ground to give the interference a return path to 0v.

[Paul P]

I tried different materials between the reverb transformer and the PT.
Nothing worked very well, probably because there was only about half
an inch between them. I found that the thicker the metal the more
effective it was, I also read this somewhere on the net. Aluminum foil
had no noticeable effect. 1/16" steel and 1/8" aluminum were a little
better and 1/8" galvinized steel a bit more. The most effective was
1/4" thick brass (these were just scraps I had lying around). Here are
some scope traces :

This is with the reverb control turned down to zero :



And turned up to max :



With a sheet of 1/4" brass between the two :



I finally bit the bullet and moved the transformer to the other side of the
tubes which put it about 3-1/2" away from the PT. I had to completely
remove the reverb board to do this so was pretty happy that I'd used
separate boards for the different circuits. And that I'd left a little extra
length to the tranformer leads.

This completely solved the problem.

Here are two traces with the reverb tone control set to each extremity :



Now if I could just figure out why my tremolo channel is not as loud as it
should be (seems to work perfectly otherwise) I'll be done.

Paul P


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