6G15 cathode bypass cap - schematic error?

[tubeswell]

Does anyone know if the "250-6" cathode bypass cap value for the recovery stage shown on the 6G15 Schem is an error? (Should it be 25uF?) (or is it 250pF?, which I think is unlikely given that there is a '6' (meaning 6V) shown?)

[martin manning]

Weber's version also has 250uf 6V in that spot. Perhaps the idea is to bypass Rk down at the reverberation frequencies (<10 Hz), although it seems like a 100uf would have done it...

MPM

[tubeswell]

Well I guess that makes sense. Silly me - I should've checked other schematics before asking. Thanks Marty. I can't source 250uF electrolytic caps that would be small enough, but I can get 100uF (bipolar) so maybe I'll go for that

Cheers

[Sir Cuitous]

It may simply be that the size of a 250uF is smaller at 6V. I believe that Sprague TVAs come in 12V (TVA1161) and 25V (TVA1208) @ 250 uF and might be a reasonable size. I am currently sourcing parts for converting a broken Fender SK100B head to a 6G15 unit and plan to try the TVA1161 (D=0.375" x L=0.812").



I would be curious to know if a 100uF sounds different.

[tubeswell]

Hi Sir Cuitous - are you intending to include any ground-loop busting measures in your unit and if so what?

[Sir Cuitous]

I'm glad you asked Tubeswell. I know you have gone over ground-loop busting in previous posts. Did Enzo ever send you a scheme for a ground-lift box?

My plan was
A) nothing ...or
B) Use a DI Box with balanced 1/4" output and/or ground lift switch.

You may have some better suggestions for me. What can I build into the circuit to lift the ground?

[tubeswell]

Hi Sir Cuitous

Well apart from rigging up a guitar patch cable to go from the output to the amp input that only has one of the shield ends connected, what else is there? (Hate to think this might be my only option)

Enzo never got back to me about the 'little box' he talked about. (Is it just a DI box?)

I was thinking of trying the ground return circuit lift that they do in the RI stand alone unit (in the location Enzo was talking about), however R.G. seems to want to discourage that (Did R.G. misunderstand the schematic? or just get it confused with my initial suggestion about elevating the mains earth connection from the chassis? He never said any more about it). What kind of diodes are they? Do you know?

I just remembered that Hoffman mentions a mod that Tore-T did on the revibe that eliminates hum, but how you apply that to a standard 6G15, I'm not quite sure. I've just cooked up a schematic for debate under that thread you were lookin' at. ;-)

http://music-electronics-forum.com/s...ead.php?t=9660

[Enzo]

I'm sorry, I didn't realize I left someone hanging at some point.

A ground loop forms when two pieces of gear have different ground potentials, and are then connected together by a signal cable. The difference in chassis voltage causes current to flow through the ground in the cable and that adds to the signal inside it.

Lifting the eaqrth ground on either piece can break the loop, but is not recommended. That leaves the signal cable. You can break the shield, but that doesn't always work, you are relying on the earth grounds back through the power strip to connect the systems.

SO yes, my little solution is more or less a direct box. I use a 1:1 transformer in a shielded box, and insulated jacks. Signal passes through without direct connection.

[Sir Cuitous]

Thanks for the succinct explanation. Is your solution as simple as it sounds?

Could that be implemented in the output section of the reverb unit if so desired? What does the 1:1 transformer add to the functionality?

I hope you don't mind a few more questions.

[Sir Cuitous]

I did find this on my first internet search (key: unity transformer). I also found this schematic.

Internet searches are notoriously bad using numbers and colons, so I just guessed that it might be called a unity transformer. Enzo, can you comment on the schematic? Are the resistors necessary for the application were are talking about?

[Enzo]

The transformer is the whole point - isolation. The 1:1 just means input and output are the same level. We are breaking a signal cord in half and inserting this thing. We want it to be transparent. We don;t want the signal level chaning from one end to the other. No change in impedance, no change in anything. Just no direct connection between the two ends.

1:1, unity, isolation, all mean the same thing.

If you made the output of your device through such a transformer and isolated (insulated) the output jack from the chassis, then sure you could incorporate it into the equipment.

The schematic you link is for stepping the speaker level signal down to size then matching it to the transformer. I have no idea the impedance ratios of the transformer they used.

My device is a transformer in a box - period. works either direction of course. Metal box for shielding exposed wires. Jacks are plastic for insulation. Depending on the project, sometimes I mount a jack on one end and a short cable wire coming out the other with a plug on it, instead of a jack at each end. Whatever is convenient.

[tubeswell]

Hi Enzo

(Pardon the pun - but in order to pry the proverbial black box a wee bit further open to illuminate the innards) for guitar amps, is there any spec on the audio transformer - that you would use in your box - that is 'better' or that you prefer over others (i.e. e.g.; size/frequency handling/dc resistance - that sort of thing?)

[Enzo]

I didn't think about it first time I did it. I got a box of small transformers from Mouser or Allied or someone, I think they might have been 15k, but the experiment worked well enough for guitar, so I searched no further. Might have found something with more low end, but it was for a guitar. And I might have found something with more top end, but it was for a guitar. I think I have two or three left. If I use them up I'll have to figure out what I used or find some suitable part.

I looked for audio isolation transformers - which would be under audio interstage transformers I suppose. I wasn't too concerned with impedance, but I did rule out things like 600:600 ohms. Whatever high impedance I could find in a 1:1.

Nothing special, no secrets stuff, no support circuitry, just a small transformer in a metal box wired to jacks.

If some demanding app came up I might look to Jensen or someone.

If I had to I could take a couple of those little mic adaptors apart. Take out the transformers and wire a couple low impedance sides together leaving a high Z on either end showing. I am referring to those little XLR to 1/4" adaptors for converting HiZ to Low Z.

What is important is that whatever goes in comes out the other side. The typical direct box tranny wouldn't work because it is usually hi to low Z. But again, two of those back to back would work.

[tubeswell]

'K Thanks Enzo

[Sir Cuitous]

Enzo thanks. Jensen is close by to me. Your explanation makes me confident enough to build isolation into my project.