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Wreck Express questions for clone builders

9/25/2000 12:42 AM
Mark Abbott	Wreck Express questions for clone builders i think the best way to address the various question regarding this amp is stage by stage. The input stage of this amp is fairly standard in various versions, though some people felt the need to use a grid stopper resistor, what purpose did the grid stoppers serve? Gris stopper values used;(33K in the A5 lite, 68K in the A3, 47K in the A4 version and 47K and a 1M trim pot for grid leak.) The option for the tone stack seem to be either using a 500pF cap with a 250K pot, or a 50pF cap with a 1Meg pot, the later being used for blues sounding amps. I am curious about the variations of midrange controls, the mids control is either a 25k pot wired as a variable resistor, with the 0.022uF cap on the hot side of the pot or the hot side of the pot attached to the bass control and the mid cap attached to the wiper of the mid control, what is the tonal difference between these two circuits? I am curious about the 820ohm resistor after the volume control, what purpose does it serve? The next area of tweaking is the coupling cap after the tone control recovery stage, and following grid leak resistor. Once again from the circuits I've seen this too is divided into two catagories, firstly a 0.001uF cap with a 56K resistor, or a 0.002uF cap and a 150K resistor. What is the difference in tone between these two circuits? The driver circuit was also an area where some people chose to use 0.1uF for input and output coupling, while other went for 0.022uF caps, was this driven by bass response problems, or tonal response? The power supply seemed standard, I'm curious here too as some of the resistor values seem quite large, is this as per the original design, or chosen for tonal response? Why wasn't a choke used in the power supply circuit? Okay thanks for reading all the questions, your replies are very much appreciated. Yours Sincerely Mark Abbott.

9/25/2000 12:42 AM

Mark Abbott

Wreck Express questions for clone builders

i think the best way to address the various question regarding this amp is stage by stage.

The input stage of this amp is fairly standard in various versions, though some people felt the need to use a grid stopper resistor, what purpose did the grid stoppers serve?
Gris stopper values used;(33K in the A5 lite, 68K in the A3, 47K in the A4 version and 47K and a 1M trim pot for grid leak.)

The option for the tone stack seem to be either using a 500pF cap with a 250K pot, or a 50pF cap with a 1Meg pot, the later being used for blues sounding amps.
I am curious about the variations of midrange controls, the mids control is either a 25k pot wired as a variable resistor, with the 0.022uF cap on the hot side of the pot or the hot side of the pot attached to the bass control and the mid cap attached to the wiper of the mid control, what is the tonal difference between these two circuits?

I am curious about the 820ohm resistor after the volume control, what purpose does it serve?

The next area of tweaking is the coupling cap after the tone control recovery stage, and following grid leak resistor. Once again from the circuits I've seen this too is divided into two catagories, firstly a 0.001uF cap with a 56K resistor, or a 0.002uF cap and a 150K resistor.
What is the difference in tone between these two circuits?

The driver circuit was also an area where some people chose to use 0.1uF for input and output coupling, while other went for 0.022uF caps, was this driven by bass response problems, or tonal response?

The power supply seemed standard, I'm curious here too as some of the resistor values seem quite large, is this as per the original design, or chosen for tonal response?

Why wasn't a choke used in the power supply circuit?

Okay thanks for reading all the questions, your replies are very much appreciated.

Yours Sincerely

Mark Abbott.

9/25/2000 11:01 AM

BWilliamson

quote:
"The input stage of this amp is fairly standard in various versions, though some people felt the need to use a grid stopper resistor, what purpose did the grid stoppers serve?"

Grid stopper there to help stabilize the amp, sometimes problems can arise from amps without this. Figure, no problems in studio conditions with short cords. Longer cords in uncontrolled conditions might cause problems with noise picked up. I don't believe KF used this, then again. I don't think people are out touring the country with his amps. Wrecks are mainly used in studios IMHO. Went with a 33K on my clone, R.Aiken from a post long ago suggests not using anything less than a 47K for this position(some math thing). Steve mentioned he likes the 47K since it shaves off a little highend off the front.

quote:
"The option for the tone stack seem to be either using a 500pF cap with a 250K pot, or a 50pF cap with a 1Meg pot, the later being used for blues sounding amps."

I went with the 500pf/1M myself. I haven't got up the gumtion to give any others a shot yet. but very happy so far. This is probably just a tune to taste thing.

quote:
"I am curious about the variations of midrange controls, the mids control is either a 25k pot wired as a variable resistor, with the 0.022uF cap on the hot side of the pot or the hot side of the pot attached to the bass control and the mid cap attached to the wiper of the mid control, what is the tonal difference between these two circuits?"

I don't know, tune to taste. I followed the original schem here myself.

quote:
"I am curious about the 820ohm resistor after the volume control, what purpose does it serve?"

A little attenuation, I don't believe this is present on a real Wreck. I believe this is a legacy from the cloner the schem was copied from. I didn't use it myself.

quote:
"The next area of tweaking is the coupling cap after the tone control recovery stage, and following grid leak resistor. Once again from the circuits I've seen this too is divided into two catagories, firstly a 0.001uF cap with a 56K resistor, or a 0.002uF cap and a 150K resistor.
What is the difference in tone between these two circuits?"

I found the .001/56K to bright sounding--not enough ummpf for me. Think this would be a really good lead tone, not much for chord work IMHO.

.002/150K--much better for me. I might even go more. Don't get stuck on the resistor value either, use a smaller resistor and a pot in series--really helps dial in a better tone. I found I like about 139K with the .002.

quote:
"The driver circuit was also an area where some people chose to use 0.1uF for input and output coupling, while other went for 0.022uF caps, was this driven by bass response problems, or tonal response?"

I'd have to agree with your assesment. My clone I went with .1uf throughout--and don't have any problems. If my (b)ass was that tight I'd get all the chicks. I think voltage might play alot with some of the things going on--this also applies to the r/c circuit before the 3rd stage.

quote:
"The power supply seemed standard, I'm curious here too as some of the resistor values seem quite large, is this as per the original design, or chosen for tonal response?"

the early version are from a source that confirmed that is what is going on.

quote:
"Why wasn't a choke used in the power supply circuit?"

Don't thing $$$ was a issue here, I'm going with KF liked or was going for a certain response from this amp. A resistor doesn't smooth out the ripple as much as a choke would. Somebody who knows better could explain this one.

Most of these are just opinions and worth nothing more than that. I'm runing 360v on the plates of the el-34's drawing about 38ma a piece. With 12AX7 had more gain than I could handle--sounded great but I swapped tubes around and ended up with a 5751 in the first pre's slot for the 1st and 2nd stage. A 12AU7 for the 3rd stage into a 12AX7 driver. Ended up looking like a Allesandro front end--sounds great cranked, and back off the volume it cleans up beautifully.

whew--bw

9/27/2000 12:50 AM
Mark Abbott	Dear Brent Thanks for your reply, as always your very helpful. May years ago I experimented with a variac and my Fender Bandmaster. When the voltage was (390V-ish on the plates of the O/P) lowered the amp had more mids and broke up easier, when I increased the plate voltage to 500V the amp had more headroom and was brighter sounding, I attribute this to the output stage mostly. I did a similar experiment with a single 12AX7 triode stage. I remember Kevin O'connor saying that changing the plate voltage from 150V to 300v makes little difference to the output voltage. I inputted a 1Khz signal and found this to be the case. This is ofcourse old news to the experts here, (not that I'm anywhere near that status). I'm trying to collect all the info possible so I can build a "successful" prototype, all advice is very much wellcome. I will be building a prototype using New Sensor transformers for a Princeton amp, people have said this amp is very sensitive to the choice of parts. Is this purely tubes, or caps, or CC resistors? Thanks for your help. Yours Sincerely

9/27/2000 12:50 AM

Mark Abbott

Dear Brent

Thanks for your reply, as always your very helpful.

May years ago I experimented with a variac and my Fender Bandmaster. When the voltage was (390V-ish on the plates of the O/P) lowered the amp had more mids and broke up easier, when I increased the plate voltage to 500V the amp had more headroom and was brighter sounding, I attribute this to the output stage mostly.

I did a similar experiment with a single 12AX7 triode stage. I remember Kevin O'connor saying that changing the plate voltage from 150V to 300v makes little difference to the output voltage. I inputted a 1Khz signal and found this to be the case.

This is ofcourse old news to the experts here, (not that I'm anywhere near that status).
I'm trying to collect all the info possible so I can build a "successful" prototype, all advice is very much wellcome.

I will be building a prototype using New Sensor transformers for a Princeton amp, people have said this amp is very sensitive to the choice of parts. Is this purely tubes, or caps, or CC resistors?

Thanks for your help.

Yours Sincerely

9/27/2000 12:58 AM
Mark Abbott	No brightness switches? I have looked at all the Express photos at the Trainwreck web site, and they all have bright switches, I think maybe Peter S ( who has tried one of these amps) could explain the effect of the bright switches, and if the amp became too bright. Mark Abbott.

9/27/2000 7:32 AM
Steve A.	Mark: One cloner found the bright switch to be handy once he cranked up the volume a bit... Maybe if you use the 0.1uF caps throughout the PI it becomes more important to use the 3 position bright switch (I think that 100pF and 500pF were mentioned). BTW there have been some reports that at least a few of the Trainwrecks used an RC network to bypass the tone stack... no values were mentioned so you'd have to experiment around. --Good luck! Steve Ahola

9/27/2000 7:32 AM

Steve A.

Mark:

One cloner found the bright switch to be handy once he cranked up the volume a bit... Maybe if you use the 0.1uF caps throughout the PI it becomes more important to use the 3 position bright switch (I think that 100pF and 500pF were mentioned).

BTW there have been some reports that at least a few of the Trainwrecks used an RC network to bypass the tone stack... no values were mentioned so you'd have to experiment around.

--Good luck!

Steve Ahola

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