6G15 in Silvertone reel to reel amp chassis?

[Toxophilite]

HI
I'm making a 6G15 reverb unit for a friend of mine in exchange for some time mixing an album
(he would actually do the mixing for free but he really wants one of these and I'd like to give him something for his time


Anyway I have a cute Silvertone reel to reel chassis that seems to have the right sort of transformers
i'll probably replace all the relevant tube sockets and relocate the 9 pin ones and put in a section of hammond coneboard to build the circuit on. Maybe I can even try to hook the funky old round VU meter into the circuit for kicks


the transformers seems to be still working but I was curious if their orientation right on top of each other will be a problem


I know in the original unit the PT was on the inside and OT on the outside
this one is the opposite.
I made one of these before with both transformers external and it wasn't a problem but they weren't right on top of each other
(I'm currently 1/2 way through removing all the moldy old circuit)
What do you think?

[sluckey]

I think it's doable. Might have some fun challenges. Apparently the transformer orientation didn't bother Sears. Keep in mind that Sears consumer electronics were never held in high regard.

[PRR]

> orientation

Audio OT to drive the reverb tank?

Reverb tanks do not pass 60Hz hardly at all.

[Toxophilite]

Cool
I guess i"ll find out and i can always move the OT if it becomes a problem
I was under the impression the 6G15 is essentially a  little single ended amp driving a reverb tank instead of a speaker, Is that not correct/
i think on the hammond site the 6G15 tank driving transformer is the same as the champ OT
The previous 6G15 I made was convereted from a Garnet reverb unit but I replaced the Garnet OT (tank driving transformer) with a German single ended reel to reel audio OT and it seemed to work
I needed to replace it because the original Garnet one wasn't the right specs for the fender style tank

[eleventeen]

It has always been somewhat of a mystery to me how the controls of a 6G15 locate the output signal so close to the input. But I know better than to argue with Leo.

[sluckey]

I was under the impression the 6G15 is essentially a  little single ended amp driving a reverb tank instead of a speaker, Is that not correct/

That's partially correct. The reverb driver is a SE amp. It also has a reverb recovery amp, cathode follower buffer for the dry signal, and a passive mixer.

[Toxophilite]

Right, I was being a bit too simplistic  
I was responding to what PRR said about the OT
i always though it was neat that you could test that part of the int with a speaker

[HotBluePlates]

... I was responding to what PRR said about the OT ...

I'm gonna guess that PRR meant the reel-to-reel's original OT is "too good" (large, heavy core for bass response at power) for a reverb driver.

You could have the same power through-put on a smaller OT core if you accepted shaved-bass, which might also be good in that your reverb is less likely to sound boomy or muddy.

London Power commissioned Hammond to make small push-pull transformers for reverb driver duty, which reduces core size & weight even more. The Reverb transformer is smaller than chalk for your pool cue. Too bad that many will shy away from the pcb mount of the package...

Then again, you gots what you gots, and it means not spending $$ on a reverb driver transformer.

[PRR]

> guess that PRR

No.

What happens _if_ 60Hz hum gets INto the tank? Practically nothing! 60Hz won't go through a tank any better than it goes through a 4-inch speaker. Small-cone radios could have atrocious electrical hum and it was not obvious a foot away from the small speaker.

Frankly, if it was good enuff for a tape recorder (even the cheap ones were not cheap), then it is likely completely fine for a reverb.

[Toxophilite]

Cool
I thought it would do the trick...mind you I don't have the speaker anymore as it was pretty nasty but I can always measure the turns ratio to figure out the impedance it'll want to see with a 6V6 (I might have a 6K6 kicking around too)


It worked last time I used one for my first 6G15 build

[Toxophilite]

Hey
How necessary is the choke in the power supply of a 6G15 build?
I'm building this one on the cheap
My PT and OT measure good
The only chokes I have are a couple of 1 henry chokes from the power supply of an Hammond AO-63

[sluckey]

The only chokes I have are a couple of 1 henry chokes from the power supply of an Hammond AO-63

I would use both od those connected in series just like the AO-63. Perfect project to use those chokes.

BTW, how do you know they are 1H?

[Toxophilite]

Thanks that's good to know and I can put those little guys to use. I got them from my friend when he pulled them from his chassis to replace them with a more skookum choke


Well I could be remembering incorrectly but I'm pretty sure I had my brother-in-law test them with his fancy meter that tests inductance(he's an electrical engineer).

Using them in series like they were originally was what I was thinking too!...now where the *&^*&^% did I put the other one...

[Toxophilite]

Another question, besides hooking it's light up to the heaters so it lights up in a cool way
Is there any way I can get the VU meter to have a function in the 6G15 circuit? or even get it to respond a bit for visual effect
Not at all necessary but I thought it could be fun
here's the original schematic, It was a Silvertone 7074 reel to reel amp
Looks like it was using a 12au7 as the meter amp

[sluckey]

I'd just use that meter circuit. Connect the input of the circuit to the signal chain, maybe the output jack.

[Toxophilite]

..hahahaha  ah heck!
I nuked and paved!  Ah well I always was a little hasty

[sluckey]

Not much to it. You could always just rebuild it.

[Toxophilite]

It's true..except I yanked and ditched the small coil!

[Toxophilite]

Hey what-do-ya think?


Considering that the RCA jacks and foot switch jack will be in the centre of the back panel like the original unit


I was going to tuck the heaters in the back top corner oooor should I run them flying above the sockets in classic fender style

[Toxophilite]

Okay this build is nearing completion (note military missile type cover on power switch from the Enterprise movie set for Star Trek)
Just have to mount the pots and wire them in


I have a question about dropping resistors


With the power supply built exactly like a 6G15


PT- rectifier - 40mfd filter cap - choke - 40 mfd filter cap - (OT and 6V6) - 10K dropping resistor - 40mfd filter cap - (all the 'preamp' tubes)


I get 370vdc going to the 6V6 and the OT  and about 310 going to the 'preamp' tubes after the 10k dropping resistor


These values are bit high (they should be 295 and 250 respectively) so I was experimenting with adding another dropping resistor between choke/40mfd filter cap choke and the 6V6 /OT


15 K seems to put me in the ballpark I'm looking for (around 293 and  248  close enough)


my question is:


normally there seems to be a filter cap associated with each dropping resistor, so if I add an extra dropping resistor between the choke /filtercap and OT /6V6 do I need to add another filter cap at that point?

[sluckey]

normally there seems to be a filter cap associated with each dropping resistor, so if I add an extra dropping resistor between the choke /filtercap and OT /6V6 do I need to add another filter cap at that point?

yes

[Toxophilite]

Thanks once again!


I'm tracking down a 120hz hum/buzz


My Pt has the usual 2 taps for B+ (red wires) plus a center tap( red and yellow wire)
 So I was using a halfwave SS rectifier
I know the original unit uses 3 diodes in a row but seems to have a Pt with one red wire
I was also looking at the webber design of this unit and it seems to use a bridge full wave rectifier


Would that be a better bet? or unnecessary overkill


also if I add a filter cap can I get away with using one of the unused 20 mfd sections in my can cap or do I need to use 40 mfd like the other 3 original filter caps?


Thanks

[sluckey]

Tell us what the measured voltage is across those two red wires.

Removing hum from a full wave rectifier is easier than removing hum from a half wave rectifier.

If I had an unused 20 in a can I'd use it ***IF*** the voltage rating is sufficient.

[Toxophilite]

598 volts AC acros the 2 red wires
298 from one of the red wires to it's centertap


The Can cap is one of those 40 20 20 20 JJ can caps you pointed me at a while ago 500 vdc
I'm currently using the 40 and 2 of the 20s tied together  so i have one free 20mfd section


I'm also using another 47 mfd  filter cap for the 'preamp" supply (I know it's not all preamp, I'm just calling it that for ease) 

[shooter]

I'm tracking down a 120hz hum/buzz

I just went hunting for that same beast in my last fix.   I started with only the PA tubes in and measured AC at each PS node.  Got like 4VAC at "A", .5VAC at "B", .06VAC at "C".  Added tubes 1 at a time and spiked my "C" node, swapped that tube and all was well.
Might help, might not

do you know what your meter is, like 0-1ma, or 0-1mV?

[Toxophilite]

Cool
todays dumb question issss
 if you have a PT with a centertap on the high voltage lines...where do you attach it?? and or do you??
eekk very dumb i know. I'm pretty sure it was originally tied to ground and that's what I was doing


Also
I built a bridge rectifier


I was also, with this build, trying a ground thingy where you separate the ground from the chassis(not the earth ground)
via a 16 ohm resistor and a pair of diodes in parallel but reversed polarity
Can I tie my bridge rectifier ground to that


I had the unit working great and quiet but then my PT was getting hot...HOT!!
Like really bad, smelly hot..turned it off right away
Something is hooked up wrong
I've been testing hot wires for continuity to grund etc
Al my caps are brand new, unless I've cooked one


Yanked the PT and tested it with the variac
Voltages test good
Then I put a 100 watt light bulb inline on the hot line form the variac. It didn't light up at all (from what I read this is a current draw test/limiter)

[Toxophilite]

oh ! figuring out my own problems.so far


Bridge rectifier combined with centertap makes for unhappiness


Go the unit working
a bit of high buzz but it is upside down with wires hanging out etc
Giant reverb of course!

[sluckey]

If you connect the two red leads to a bridge you will have about 848VDC out! You can connect the bridge to a red and red/yel wire and get 424VDC.

Or, you can do a simple 2 diode full wave conventional rectifier (red/yel connected to ground) and get 424VDC.

You should probably stick with a half wave rectifier (like the original) since your PT puts out so much voltage.

[Toxophilite]

Thanks that's what I ended up doing and the power supply seems to be good despite my toasting the PT...twice.


Oh and I also could only find one of those little hammond A0-63 chokes..maybe I don't have enough filtering?

The reverb is working well I believe though it does seem to be brighter than I remeber my last one being, perhaps with some distortion in the top end


there's also some bzzz going on. Not low hum , definitely a higher bzzzzzzzzzzz, kind of irritating
So some trouble shooting is in order
As usual any and all ideas appreciated
Here's what it looks like in it's rough and ready finished state. Vintage military reverb unit...


I will get some voltages up and go over the schematic/build again.