6G15

[tubeswell]

Started my stand-alone reverb build yesterday avo with bending and riveting up the chassis out of 14 gauge aluminim and making the holes. But since it looks like all the other amps I've built, I don't need to post any pics of that part do I? My cab will be a tad higher and (I think) narrower that the stock 6G15 so I can stand it on top of my tweed style amps.

I decided not to do the re-vibe thingy at this stage after evaluating one of Mr Hoffman's clips. (No disrespect intended to Mr Hoffman. However I wonder whether maybe that harmonic vibrato is meant to be inserted in the signal chain after the first stage(s) of an amp. It seemed (to my ear) to be a bit shrill and 'frequency-comb"ish in the Hoffman recording. It probably would go good with a Bo Diddley-themed rythym)

[Diablo]

I'd like to see any pictures you might have, as I'm thinking a stand alone reverb from Weber may be my next project. Is there a difference between the 5g15 and the 6g15? I'm thinking of ordering mine in tweed to match a tweed bassman.

[Sir Cuitous]

I'm still getting parts together for mine, but you are inspiring me to get started faster.

I do have a question for anyone who might know. What is the input/output impedance of the 6G15? I know the impedances for the tank I'm using, but is that the same as the impedance of the entire unit?

[tubeswell]

I'm thinking a stand alone reverb from Weber may be my next project. Is there a difference between the 5g15 and the 6g15?

AFAICT the Weber version is pretty much a straightish copy of the Fender one (except the Weber one has a FW rectifier and a 6V6 reverb driver) whereas Mr Weber maybe named his one '5something' because his other kits are mainly '5somethings' (who can tell but Mr Weber?)

Mine will also have a FW rectifier and a 6V6 driver tube (so does that make it more of a '5G15'?). I'll post some pics later

[tubeswell]

What is the input/output impedance of the 6G15? I know the impedances for the tank I'm using, but is that the same as the impedance of the entire unit?

Hi Sir Cuitous

Which part of the circuit do you mean? The reverb tank send stage output impedance and tank return stage input impedance? I'd have to sit down for a few minutes and work that out.

Whatever it is, I'm building mine (sort of) stock, but with a 6V6 reverb driver going into a champ style OT that has a nominal 5k reflected load (I chose this value based more-or-less on the OT in the Tore revibe schematic (attached), and I happen to have a 5k-8R SE OT lying around). I'm also using an Accutronics 4AB3C1B pan (8R input impedance, 2.25k output impedance)

As far as loop-busting is concerned, I thought I'd try Enzo's suggestion of making up an isolation box with a small audio tranny, but haven't made up my mind yet whether to mount this inside the Reverb unit chassis or not (at this stage). I assume such a component would be better being shielded in a little project box with shielded cable connections going to an isolated output jack, even if it were inside the main chassis.

[Sir Cuitous]

Tubeswell, I meant the impedance of the whole thing jack to jack.

I also want to go with a FW rectifier, but I am little vague on how that will work. (I like having a picture/schematic to look at).

I am going build the isolation tranny into the unit. I spoke with a tech from Jensen Transformers today hence the impedance question. I really need the output impedance at the output jack. I am assuming that it is not the same as the output impedance at the tank output. The tank I have is the Accutronics 4AB3C1B which has an output impedance of about 2250 ohms. I got the tank for free from a friend. I am converting a Fender SK100B head to a reverb unit, and will mount the tank at the bottom of the cab.

[tubeswell]

6G15 Scheme with output isolation option

I also want to go with a FW rectifier, but I am little vague on how that will work. (I like having a picture/schematic to look at).

Hi Sir Cuitous

I recall reading somewhere that it is better having a lower output impedance feeding into a higher output impdeance, so the higher the impedance of the isolation tranny the better (I think?)

Anyway I had a go at a schematic for how it might look (see insert for isolation tranny option).

The voltages are what I would aim for from the Fender schematic (in order to keep the current draw down so I don't blow the Choke, which I think is rated at 50mA, but some sources say 40mA, anyway I have some spare chokes in case). In order to do this I am thinking to maybe zener down the B+ or use smaller value (~22uF) filter caps

With a 240-0-240 secondary FW diode rectifier I estimate I'll get about 340VDC B+, which might drop a bit under load to about 320V, which I can zener down if necessary.

I think (by way of reasoning that you don't get a 70V drop through an OT primary) that the Fender Schematic I'm looking at has a plate voltage of 285V for the 6K6 (not '225V', though its very hard to read). So therefore in expecting somewhere around 330V tops for the 6V6 plate, which might mean 27-28V tops at the cathode, that's about 28mA max tube current (running in Class A), and with the 12AX7 and the 12AT7, that makes about 40mAish. Whaddaya reckon? (Anyhow, the Tore schematic looks as if it is designed for around these voltages)

[Diablo]

AFAICT the Weber version is pretty much a straightish copy of the Fender one (except the Weber one has a FW rectifier and a 6V6 reverb driver) whereas Mr Weber maybe named his one '5something' because his other kits are mainly '5somethings' (who can tell but Mr Weber?)

Mine will also have a FW rectifier and a 6V6 driver tube (so does that make it more of a '5G15'?). I'll post some pics later

I've got some 6K6 tubes to use. Will it make a difference with the 6V6?

[TD_Madden]

Use the 6K6,,,there is a noticeable sound difference.

[Sir Cuitous]

I have a Mojo PT. The secondary is 250-250 with no center tap. Does that limit my rectifier choice to HW? I wont cry if so. I suppose I should read up about rectifier circuits. I have a fair understanding of tube rectifiers, and a glance at a few diode circuits but that's about it.

Tubeswell, I very much appreciate providing a schematic. I agree with you about the voltage drop. It sure looks like a 70v drop on the Fender schematic.

It appears that I also need to get more of a handle on impedances. I have always thought that the I/O impedance of each amp stage and the OT should match as close as possible. Building from tried and tested designs as in the re-creation of old Fender amps has allowed me to ignore impedance matching. If I intend to do any significant mods, I will need to learn more.

I can certainly source the isolation transformer to be as high of impedance as possible. The Jensen tech said that an output impedance of over 1000 ohms could not be isolated with a 1:1 transformer. I need to check on that. I guess my question now would be, is there a way to calculate the impedance following the reverb pan? If the impedance at the output of the pan is 2250 ohms, then what effect does the recovery stage and the tone and mixer pots have on the impedance? Could the recovery stage lower the impedance or is that a dumb question? I'm wearing my asbestos suit now.....

[tubeswell]

Use the 6K6,,,there is a noticeable sound difference.

Hi TD Madden

Would that be a 'tone' sound difference or a 'distortion/saturation' sound difference (or both) do you think?

(I have a couple of SE trannies I could use - one is larger than the other, and the larger one has an 8K reflected load with 4R, 8R and 16R taps)

[tubeswell]

I have a Mojo PT. The secondary is 250-250 with no center tap. Does that limit my rectifier choice to HW? I wont cry if so. I suppose I should read up about rectifier circuits. I have a fair understanding of tube rectifiers, and a glance at a few diode circuits but that's about it.

It appears that I also need to get more of a handle on impedances. I have always thought that the I/O impedance of each amp stage and the OT should match as close as possible. Building from tried and tested designs as in the re-creation of old Fender amps has allowed me to ignore impedance matching. If I intend to do any significant mods, I will need to learn more.

I can certainly source the isolation transformer to be as high of impedance as possible. The Jensen tech said that an output impedance of over 1000 ohms could not be isolated with a 1:1 transformer. I need to check on that. I guess my question now would be, is there a way to calculate the impedance following the reverb pan? If the impedance at the output of the pan is 2250 ohms, then what effect does the recovery stage and the tone and mixer pots have on the impedance? Could the recovery stage lower the impedance or is that a dumb question? I'm wearing my asbestos suit now.....

That PT should be in the ballpark with a bridge rectifier. You could always put a variable voltage regulator in the B+ line to knock it down a bit (There was a thread a couple of days ago by Daz about this under theory and design I think, and Bruce knows about that sort of stuff).

As to circuit impedance, I need to do some more reading up too

[Sir Cuitous]

Yes, a FW bridge rectifier. I have looked at some diagrams online and I will design for that. Thanks for pointing me in the right direction. As for impedance, I'm going back to the books and searches to see how much I can learn before choosing the isolation transformer. For voltages I try to avoid zeners and regulators. I feel if I can match the transformers to a project then I'm going to be in the ballpark. Theoretically, I can also increase the cathode resistor of the 6K6 if the bias current is too high, right?

If anyone is interested I can share my isolation transformer selection once I have made a choice. What a great community here. Thanks for helping me out.

[TD_Madden]

Hi TD Madden

Would that be a 'tone' sound difference or a 'distortion/saturation' sound difference (or both) do you think?

(I have a couple of SE trannies I could use - one is larger than the other, and the larger one has an 8K reflected load with 4R, 8R and 16R taps)

I think you hit on the the head...it's a bit of both. I had an Uncle Spot unit some time back with a 6K6 stock. I swapped it out for a couple different 6V6 model and always went back to the original.

[tubeswell]

Hi Sir Cuitous

I'm keen to see what you come up with in the way of specs for your tranny selection.