What to tweak?

[Tdale]

If you for some reason feel like e-mailing some de-gooped pictures, feel free to use my e-mail address

tommy.dale@c2i.net

Tommy

[pedro]

really interesting Scott / Guys...

so far my build has 4.7uf on CL1 and 1.5uF on CL2 - I'm going to tack 22uFs on both and see what impact.

I had prev thought part of the grail sound was from low cathode bypass but this seems to point the other way. Ill prob make the cath bypass switchable to see the difference in the "experimental" chassis.

Thanks for sharing the degoopoed tip btw,

- still waiting for some decent MF plate loads to arrive.
I have a stash of Welwyn glass metal oxides from an "ex-BBC radio engineering" clearout which I've been using but these are now mainly in wrong values !!!

see ya.

Pete C

[llemtt]

Thank you Scott

now it's a lot clearer to me!

..the grail sounding speciman had 25uf caps on V1a and V1b. .05/.01/.005 couplers. And importantly, the skyliner stack in combination with no post OD stack..

Can't remember how and when, but I have always known this although I own a different opinion on bypass and couplers so I build with different values: one for all .022 midcap!!!

..There are other degooped ones as well. Interesting stuff that is not in the never been gooped ones.

This is what really interest me the most.

I went into Dumbles and GOOP ten years ago just by chance because at that time I was into solidstate tube sound emulation and I was looking for info using the early internet, no forums then but lot of newsgroups.

Someone was saying that solidstate was even better than tubes and the proof of that was that Dumble amps, and this was the first time I saw that name, actually had a TubeScreamer hidden under the GOOP!! Someone then replied that it was completely false because he once removed the GOOP from a Dumble and, as we all know, there wasn't a TS inside.

But he also said that he found something very interesting instead, and very clever, at the entrance of the OD stage, something you won't find in any other guitar tube amp, although it wasn't a brand new idea, it was common "radio" stuff. HAD genius just applied this idea in a different contest.

I came finally into the D thing only 4-5 years later when I realized that solidstate is ok but tube is better! Unfortunately I had to change computer two times and I lost all those info, since then this "idea" bugs me every time I read about GOOP.

If you kindly want to share some pix and info on this subject here is my mail

teo@m-elli.it

Teo

[pedro]

Interesting - I have a link to a site ful of music electrincs schematics.
There is an alphabetical index by manufacturer of all the schems this guy has collected - and he has published them on his site.

http://www3.telus.net/public/vintage1/guitar.htm

There is an entry under dumble - with commentary that HAD had requested that the site owner remove all the data there.
The author goes on to say that there was something unique about the implementation and that he had never seen the technique used in guitar amps before. He couldnt divulge. That snippet has bugged me for years !! So close and yet so far. I wish I'd seen the original info that was posted to the site before he took it down !

Nowadays Ive come to the conclusion that the basic schematic we have is pretty close and the rest is about tuning the circuit to taste.

any other thoughts ?

Rgds all.
Pete C

[dogears]

Bullshit, since the guy doesn't even realize you can't just buy one.... I think the base circuit is pretty well known. Just because some guy did not do a good job on his clone, doesn't mean that it is because he was missing data.

Just my 2c

Interesting - I have a link to a site ful of music electrincs schematics.
There is an alphabetical index by manufacturer of all the schems this guy has collected - and he has published them on his site.

http://www3.telus.net/public/vintage1/guitar.htm

There is an entry under dumble - with commentary that HAD had requested that the site owner remove all the data there.
The author goes on to say that there was something unique about the implementation and that he had never seen the technique used in guitar amps before. He couldnt divulge. That snippet has bugged me for years !! So close and yet so far. I wish I'd seen the original info that was posted to the site before he took it down !

Nowadays Ive come to the conclusion that the basic schematic we have is pretty close and the rest is about tuning the circuit to taste.

any other thoughts ?

Rgds all.
Pete C

[dogears]

Teo,

Normster and Ben Shelton are both using .02 midcaps and 270pf treb caps. That is what was in another degooped one....

Shoot me an email please.

Thank you Scott

now it's a lot clearer to me!

..the grail sounding speciman had 25uf caps on V1a and V1b. .05/.01/.005 couplers. And importantly, the skyliner stack in combination with no post OD stack..

Can't remember how and when, but I have always known this although I own a different opinion on bypass and couplers so I build with different values: one for all .022 midcap!!!

..There are other degooped ones as well. Interesting stuff that is not in the never been gooped ones.

This is what really interest me the most.

I went into Dumbles and GOOP ten years ago just by chance because at that time I was into solidstate tube sound emulation and I was looking for info using the early internet, no forums then but lot of newsgroups.

Someone was saying that solidstate was even better than tubes and the proof of that was that Dumble amps, and this was the first time I saw that name, actually had a TubeScreamer hidden under the GOOP!! Someone then replied that it was completely false because he once removed the GOOP from a Dumble and, as we all know, there wasn't a TS inside.

But he also said that he found something very interesting instead, and very clever, at the entrance of the OD stage, something you won't find in any other guitar tube amp, although it wasn't a brand new idea, it was common "radio" stuff. HAD genius just applied this idea in a different contest.

I came finally into the D thing only 4-5 years later when I realized that solidstate is ok but tube is better! Unfortunately I had to change computer two times and I lost all those info, since then this "idea" bugs me every time I read about GOOP.

If you kindly want to share some pix and info on this subject here is my mail

teo@m-elli.it

Teo

[oldmacman]

I emailed that guy about the schematics and he doesn't have anything that isn't publicly available on Schematic Heaven or Blue Guitar.

Ben

[pedro]

fair enough.......thanks Ben,

Scott - Bullshit indeed !..........which is good - because it means there isnt any magic mojo going on which we dont already know . ( But you already knew that Scott ! )

Pete

[caglej]

But he also said that he found something very interesting instead, and very clever, at the entrance of the OD stage, something you won't find in any other guitar tube amp, although it wasn't a brand new idea, it was common "radio" stuff. HAD genius just applied this idea in a different contest.
Teo

This is interesting.....It made me wonder how the wierd OD injput network works. I know most everyone goes with the 220k/180k and 100k pot for non HRM, but what does the thing do? Is it a notch filter? Maybe those who have tried it could share their experiences and knowledge.

[llemtt]

..that there was something unique about the implementation and that he had never seen the technique used in guitar amps before..

I know that site but it seems to me it has been that way ever since, at least last 3-4 years, and it's not my original (lost) source. BTW I think the author is barely referring to snubbers from plate to cathode, nothing new under the sun...

[llemtt]

..It made me wonder how the wierd OD injput network works..

Are you referring to the 70s network? The coupling-cap/resistor/trimmer/grid-resistor network doesn't look weird to me at all.

Basically it's a bandpass filter that is also used to reduce the signal, there isn't a notch, you need a combination of T and PI filters or and inductor to build a notch (passive).

The high-pass part is made up from output-resistance of the previous stage (V1b), coupling-cap, resistor + trimmer, it's a first order filter so it has 6dB/Oct slope. Interesting thing here is that V1b output-resistance is different (lower) if NFB loop is in place.

The low-pass part is made up from output-resistance of the previous stage (V1b) + coupling-cap + resistor + {trimmer} + grid-resistor and the grid-anode capacitance of the tube (V2a) multiplied by the voltage gain of the stage (Miller effect). It's a first order filter so it has 6dB/Oct slope.

Interesting thing here is that the overall resistance depends a lot on trimmer setting because at 0 setting it is obviously 0 + 68kgrid-resistor = 68k, while at max setting it's trimmer resistance parallel to [V1b-out + coupling-cap + resistor] + 68kgrid-resistor = approx 150k considering 100k trimmer.

To define the model even better you should also include other "not so obvious" component and parameters, one for all shielded cables capacitance.

Adding some resistor bypass caps like the 47p found in some 97schema, and used for instance by some TR E.Pro, makes the model more complicated but basically acts like shelving filter enhancing high freqs.

The 70s network adds another stage to the low-pass filter (V1b-out into 220p-500p cap to ground) making it a 12dB/Oct.

Teo

[caglej]

Teo,
Thanks for the excellent explanation! I feel like it's making more sense now, but for the purpose of tweaking frequency response, what change would increase the bass rolloff frequency of the high pass portion of the filter? (If I wanted to decrease OD channel bass response) I know that this is accomplished by reducing V1b coupling cap size or by decreasing cathode cap values on V2a or V2b, but what will the change in resistance actually do to the crossover point? Which direction does increasing the resistance move the crossover knee?

Thanks,
JC

[llemtt]

Formula is f = 1/6.28*R*C so decreasing capacitance or resistance is the same: both increase the roll-off freq of the filter. Consider that resistance has also an effect on the low-pass filter, so to control the bass roll-off it's easier to tweak the coupling cap only.

The cathode bypass caps work a little differently: the filter isn't a first order low-pass but a shelf filter where the high freqs have the full bypassed gain of the tube stage while the low freqs have the unbypassed gain of the tube stage. Talking about the "freq" of the filter here is a bit misleading but you can maybe read good explanations at the aiken site!

Teo

[groovtubin]

..wondering if the DEGOOPED pix..

Hi Jim

Do you really mean DEGOOPED instead of NOTGOOPED?

I never saw a DEGOOPED Dumble, Have you? Is it so different?

Teo

[groovtubin]

really interesting Scott / Guys...

so far my build has 4.7uf on CL1 and 1.5uF on CL2 - I'm going to tack 22uFs on both and see what impact.

I had prev thought part of the grail sound was from low cathode bypass but this seems to point the other way. Ill prob make the cath bypass switchable to see the difference in the "experimental" chassis.

Thanks for sharing the degoopoed tip btw,

- still waiting for some decent MF plate loads to arrive.
I have a stash of Welwyn glass metal oxides from an "ex-BBC radio engineering" clearout which I've been using but these are now mainly in wrong values !!!

see ya.

Pete C