DIY SVT tube bass amp

[okabass]

[quote author="ramjet"]hi

that is so cool

congrats on a job well done

it has inspired me for sure

cheers ramjet[/quote]

HI
Nice to hear that. The amp has inspired me too: 'cause it sounds and feels so good.

Regards, Okabass

 

[lolo-m]

Hi, nice work !!!

I'm planning to modify an old dynacord guitar amp into a bass amp but I want to build a SVT original preamp. I've got the schematics but I cannot know what are the inductor values (it is labelled 320821-1 on the schem ).
Does someone know something about it ?

 

[plexi]

The inductor is a tapped toroid with 800mH, 300mH, and 100mH taps. I have some here if you would like one or 2 to play around with. PM me and we can work it out.

 

[walter]

I have tried to use a current limiter to check an SVT for shorts, but the bulb just shines, they do draw some current, maybe a 200watt bulb would work. Those SLM power transformers with internal fuses are made to sell transformers, just tear into the end, and bypass the blown fuse. I know a fellow named Uli that may do a version of the SVT, an exact copy only the names changed to protect the guilty, and mass produced in china.

 

[okabass]

[quote author="lolo-m"]Hi, nice work !!!

I'm planning to modify an old dynacord guitar amp into a bass amp but I want to build a SVT original preamp. I've got the schematics but I cannot know what are the inductor values (it is labelled 320821-1 on the schem ).
Does someone know something about it ?[/quote]

HI
Check Ampeg V9-pre schema. There is a simplified mid control, which works well. Do first the SVT-pre from input to the tone controls, then do that mid control from the V9-pre. You can leave the cathode follower out, if you drive a hi Z input. That's it.
http://users.aol.com/portaflex/schems/v9preamp.gif

 

[mata_haze]

Hi there,
is any of you out there that could provide some schematics of the SVT DI?
i am trying to understand how good it is and if it could be possible to do a DIY after that.
any help will be much appreciated.

 

[okabass]

[quote author="mata_haze"]Hi there,
is any of you out there that could provide some schematics of the SVT DI?
i am trying to understand how good it is and if it could be possible to do a DIY after that.
any help will be much appreciated.[/quote]

HI
I have SVT DI. I got it from swapping some things. I have not used it, cause I use a mic. I my opinion and experience make this kind of tube DI.
http://www.jensen-transformers.com/as/as091.pdf
Or:
http://www.jensen-transformers.com/as/as021.pdf

I belive the tube cascade is "the circuit" to drive transformer and line output. The SVT DI (according to manual) doesn't have a cascade but a cathode follower . As matter of fact I was planning to tweak it , but ended using a mic. Gyraf- site has also very good info. Check the G9 mic pre's output.

 

[acdc126]

okabass, on the site talkbass.com, you said "I connected one 2k/5W ceramic wire wound resistor wrong. It didn't burn and smoke: it literally exploded away" :S i cant find that 2k resistor in the schematic but it must be very bad if it explode. i did 2 errors like that when i've made my marshall jtm45, but im more lucky than you because nothing broke, it was only a bad bias too low because of a 30k resistor that i used for a filter circuit of the 6,3v filament. it was on the wrong part. i just waste my time to find why it was impossible to have more than -40v on bias and bias meter was so crazy i always turn off immediatly

other error, i forget to ground the valve 1B (preamp 2) you can imagine how noisy it was when i turned the volume of input 2.

not easy to build or modify without error. :thumb:

 

[BYacey]

[quote author="mata_haze"]Hi there,
is any of you out there that could provide some schematics of the SVT DI?
i am trying to understand how good it is and if it could be possible to do a DIY after that.
any help will be much appreciated.[/quote]

Yep: http://img.photobucket.com/albums/v354/byacey/SVTDI.jpg

 

[NewYorkDave]

Something I don't get about that schematic: why include a cathode follower for low output Z, only to ruin it by taking the output from the junction of R6 and R7? (That's also throwing away much of the gain from the first triode).

 

[BYacey]

I don't know why they would do that; I just drew it out one day while half watching TV with the wife. The DI box was far more interesting than the show. I think it was American Yodeler or some other idol worshipping kind of program.

In looking at it further, perhaps that's what they wanted to do, was drop some gain so the direct or tube output levels are somewhat matched for level. Besides, with the average guitar pickup putting out a hundred or so miilivolts with an enthusiastic player, You probably would want to drop it a bit if driving a mic pre on the end of the line.

 

[clintrubber]

[quote author="BYacey"]Yep: http://img.photobucket.com/albums/v354/byacey/SVTDI.jpg[/quote]
Thanks for posting :thumb:

Did the output-TX reveal any more specifics ? Based on the relatively high source impedance of the attenuated CF-output and the direct-possibility, it'll be a Hi-Z to Low-Z stepdown, right ?

Regards,

Peter

 

[Bryson]

Is R4 6.8 or 68k?
Why is it there (in parallel with R6)?


Or, why is R6 there?

 

[BYacey]

R4 is 6.8K, don't ask me why they made this resistor network like this. There is a possibilty I made an error drawing this out, but usually I'm pretty thorough about this kind of thing.

I didn't pay much attention to the transformer aside from noting the substantial amount of iron in the core. This was a unit brought to me for service and I just had it for a short time so I didn't spend a lot of time taking measurements.

 

[clintrubber]

[quote author="BYacey"]I didn't pay much attention to the transformer aside from noting the substantial amount of iron in the core. This was a unit brought to me for service and I just had it for a short time so I didn't spend a lot of time taking measurements.[/quote]
OK, thanks anyway. So I understand it wasn't branded.
I figure one of these Jensens can substitute
( http://www.jensentransformers.com/datashts/dbe.pdf, http://www.jensentransformers.com/datashts/dbepc.pdf ), but wouldn't know of any other or 'economy'-candidates right now.

But what am I thinking ? No new projects ! :wink:

Regards,

Peter

 

[SonsOfThunder]

Nice one Bill! I've been wanting to see this for some time. Not exactly what I expected but then again it is...

I really agree with the puzzle NYD presents. Its not like they are hitting the second section hard with the firs section, why not use a resistor ladder there to knock off the gain and keep the low out Z intact? Or just use a lower plate resistor in the first place?

As (another) back-burnered project, I have a prelim schematic for a DI I started based on the front -end of the SVT preamp section using a 12DW7. I gave a copy to a friend several months ago for him to start the work of building a proto but have not heard from him. I should just post it here and see who can pick it up and run with it.

Any chance you made any photos? :grin:

On second viewing, are you sure about the pad on the front end? Looks like it alters the input Z radically??

Thanks again for sharing!

 

[clintrubber]

[quote author="SonsOfThunder"]Nice one Bill! I've been wanting to see this for some time. Not exactly what I expected but then again it is...

I really agree with the puzzle NYD presents. Its not like they are hitting the second section hard with the firs section, why not use a resistor ladder there to knock off the gain and keep the low out Z intact? Or just use a lower plate resistor in the first place? [/quote]
Just some speculation....
who knows they liked the sound for the 'active-mode' better
when driving the TX from a not that low impedance...

... since this DI-box has a 'direct'-mode as well (bypassing the tube),
the (prim of the) TX needs to be relatively Hi-Z anyway,
so the resulting source impedance of the complete active
section doesn't have to be that low.

Which leads to rephrasing the question:
why did they use a CF then ?

 

[BYacey]

[quote author="SonsOfThunder"]


Which leads to rephrasing the question:
why did they use a CF then ?

For want of a better use for the second triode?
I do have pictures of the PC boards, I'll throw 'em up on 'Bucket.

 

[clintrubber]

[quote author="BYacey"][quote author="clintrubber"]Which leads to rephrasing the question:
why did they use a CF then ?[/quote]

For want of a better use for the second triode?[/quote]
It'd been easy to find a better use for the second triode,
for instance as an additional amplification stage for 'more tube-bending'
(add yet more attenuation :twisted: )


But hey, this box breaks at least one of the Craig Anderton-FX-box commandments:
"thou shall not polarity invert"

Enabling the active section inverts polarity w.r.t. the straight path,
a thing they could have easily prevented for instance by that second triode section.

Imagine the confusing fun that could be had in DI + mic setups because of this property...
player decides to engage the tube for his amp-feed for a touch more colour on song #3
(which results in inverted polarity of the mic-signal) while the direct XLR-out feed remains
unaltered on either one of its settings.

'Design flaw' is a big word for it, but it's at least a bit unelegant
for such a nice thing this box looks to be....

I do have pictures of the PC boards, I'll throw 'em up on 'Bucket.

Nice !

What's that with the internal lightbulb BTW ?
Harmony Central review of this box talks about some Beh.-style fake-tube-glow,
but is it perhaps just a stylish on/off indication ?

Bye,

Peter

 

[SonsOfThunder]

>>>I do have pictures of the PC boards,

AwwwwYeahBoiiiiiiiiiiii! :green: :green: :green:

>>>why did they use a CF then ?

Yes, why not just use the two sections in parallel as a CC amp for lower effective Rp and thus more drive current??