Questions about building AB763 out of Bogen amp...

[chikov]

Hello,
I just posted similar post on Fender files, but decided to move it here (but I do not know how to delete the old one).
I just finished building modified AA964 by using 1946 transformers from an old see burg amp and it sounds amazing. Being Catodyne PI it does have a lot of mid range but I like this vintage feel to it. For my next project I would like to transform my old Bogen Challenger amp that used 7868 tubes into Deluxe Reverb blackface amp, but without the Vibrato channel.
[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/46c8520e-73cd-4308-b01c-3147204a552f_zps50lscrmo.jpg[/img]

The first question I have - Does anyone have a schematic of modified AB763 (or similar amp with the long tail PI), without the Vibrato but with the reverb?

Second question is - on OT center tap Bogen has about 460V, but the AB763 about 420V. Will it work with 6V6 tubes or should I switch to 6L6 instead? I checked the transformer and the two leads on secondary show only 186V, I am not sure where the 460Vdc will come from... Here is the schematic:

http://www.makearadio.com/schematics/im ... 100b-6.jpg


The last question has to do with my reverb can. Several years ago I got it out of an old Hammond organ. Will it work for this Fender type amp?
[IMG:640:360]http://i409.photobucket.com/albums/pp171/alexjohnne/8ee77e4b-686c-4ee4-92f1-383ca361ab3f_zpsh5nigvti.jpg[/img]

[Leo_Gnardo]

For the power supply, if you stick with the "voltage doubler" circuit Bogen used, you'll have no trouble raising enough voltage to run a pair of 6L6's for all the power they can put out. You also find voltage doublers in some other classic circuits like Sound City and venerated McIntosh tube power amps.

Seems sort of funny trying to turn a potent Challenger 100B circuit into a low output power Deluxe. If you must use 6V6's, the only ones I can predict will put up with the high voltage you're going to generate are JJ's, they seem to be the toughest of the currently available 6V6's. With 6L6's you'll have something more along the lines of a Pro or Vibrolux Reverb, or Vibroverb.

You don't need a whole other schematic, just ignore the vibrato parts of a Fender schematic.

It's very handy that your Bogen has 2, 4, 8 and 16 ohm output windings. It's likely, if you use only 2 output tubes, that 2 will become 4 ohms, 4 will become 8, etc. I'd do the power amp conversion then test with load resistors to see whether this is really the case. The load that draws the most power from each winding is the correct one, that's the definition of a proper impedance match.

[chikov]

Thank you, Leo, for the help. I will definitely use "voltage doubler" ( another concept for me to learn) for this build ( I wanted to hear Diode vs. Rectifier tube difference anyway).
I will go with two 6L6s this time. Do you think building something similar to Bassman AA864 will work with this PT and OT?

http://www.davidsonamp.com/sf/images/bassmanaa864.gif

I did not know that using 2 tubes instead of 4 will change the dynamics of the OT, but it makes perfect sense. I will study it also as I go...

How hard is it to add Reverb circuit to the AA864?

PS. I do want to build the amp with the long tail PI - that is why I am doing it, plus it will be cool to use that spring reverb can that I've got.

[Leo_Gnardo]

Check that reverb tank - measure the resistance of the drive coil & pickup coil. If it is to be compatible with Fender's circuite, you'll be looking for about 1 ohm DC resistance on the drive coil and 200 on the pickup coil.

On OT's, by removing half the drive tubes, you double the drive impedance, and need to double the impedance on the secondary to match. Now you're going from one type of tube to another, that complicates things a bit, but I think you'll be in the right ballpark to use the impedance-doubling rule. Just a good idea to check & see what you've got once the power amp is assembled. That will take a sine wave generator, load resistors and oscilloscope. If you don't have these on hand, get together with someone who does. It would be a pity to go to the trouble of doing the mod & not know what your correct output taps really are.

[Smokebreak]

Why not use the 7868/7591A tubes? If you don't have them you can get new production ones. They're right in between an L6 and V6, IIRC. Then you don't have to fiddle with power amp stuff too much.
Also, if you want to best replicate AB763 w/o vibrato, don't forget to simulate the loading of the vibrato 50K pot, as well as the 220k/220K voltage divider before the LTPI. Of course these can be fun areas to fiddle with, as well.

[tictac]

Bogen... oh, OK but gutting a BOGNER to build a Fender reverb... now that's messed up...

[Leo_Gnardo]

Bogen... oh, OK but gutting a BOGNER to build a Fender reverb... now that's messed up...

Considering some of the Bogner mods I've had to sort out, gutting a Bogner to build a Fender reverb amp would be a sort of justice.

I do have respect for Bogner's amps fwiw. Not so much for his mods I've encountered.

[chikov]

I am sorry guys, it is a Bogen amp and the reason why I decided to convert it to Fender style amp is because of the cost of the 7868 tubes, plus I would have had to change a bunch of settings anyway to make it he way i like it. In my mind i am working my way through the evoulution of the famous amps. My first build was A Cathodine PI 50's style 6V6 amp. My next one is 60's style lond tail PI 6v6 Bassman ish amp. And I hope to graduate into the Dumble style amp in the end. I did build Ceriatone Overtone cpecial from their kit but I did not know then even 90% of what I know now...
Here is the tone I absolutely love:
http://youtu.be/9U1mrbqyEyw
(Did anyone here tried to clone this amp?)

I am getting ready to study up on usage Osilloscope+ signal generator + dummy load. (Does anyone have a link to the useful web page teaching this stuff?). In a little bit i will measure the resistance of my reverb tank coils too. Thank you for all your help.

[Smokebreak]

I've built what I can "guess" is inside the OD Deluxe, and some forum members have even cloned it based on having possession of that exact amp, and have done a great job at it, to boot.

JJ 7591(same as 7868) are on sale for $14 a bottle. https://tubedepot.com/products/jj-7591s ... oCJbnw_wcB

[chikov]

I've built what I can "guess" is inside the OD Deluxe, and some forum members have even cloned it based on having possession of that exact amp, and have done a great job at it, to boot.

Could you post a link to your thread or schematic please?


Here is the photo of an old Navy, military oscilloscope I've got that still works.

[IMG:640:499]http://i409.photobucket.com/albums/pp171/alexjohnne/963ff6f4-7ab7-4a33-9d4d-95b54bb4dd31_zps2wknzvil.jpg[/img]

will it work for our type of tube amps?

I have to admit that I was never taught how to use them, but I am willing to learn and I am a fast learner.

[Smokebreak]

Could you post a link to your thread or schematic please?

Sure : https://tubeamparchive.com/viewtopic.ph ... ght=deluxe
I based this on some pictures of a different "Overdrive Deluxe", that had what seemed to be a 3 position rotary. Turns out the one from the videos is a single channel amp.
My amp took a turn here and a turn there, as I was just having fun, and really into the idea of overdrive sounds derived from Fender topology.
But all in all, I based it on a second/third generation Dumble. You can find the schematic in the Dumble Files section. Let me know if you have any questions about that.

[chikov]

Check that reverb tank - measure the resistance of the drive coil & pickup coil. If it is to be compatible with Fender's circuite, you'll be looking for about 1 ohm DC resistance on the drive coil and 200 on the pickup coil.

Yes, I do get about 1 ohm on the drive and about 183 ohms on the pick up coil.
I am getting ready to put together parts to start AA864, but I hate that V3 uses only 1/2 of the tube... I wish I could use the other half as well... May be I can add the reverb part from AA763 to the AA864 thus use the other half of V3 for recovery stage... I need to look into it...

[Phil_S]

... V3 uses only 1/2 of the tube... I wish I could use the other half as well...

It's not that big a deal. If you are having trouble getting past it, get a 6AV6, which is the equivalent single triode tube to a 12AX7.

[Leo_Gnardo]

I hate that V3 uses only 1/2 of the tube... I wish I could use the other half as well... May be I can add the reverb part from AA763 to the AA864 thus use the other half of V3 for recovery stage... I need to look into it...

What other half of V3? The Bogen schemo shows a 6C4, which is a single triode, used as an output drive tube.

If you're going to make a more or less Fender reverb amp I could see putting an another tube socket for a dual-triode Fender style output drive and nicking that 6C4 for a reverb drive: I think it would work fine with a Fender style reverb drive transformer, as its triode is pretty much the same as one in a 12AU7A.

[chikov]

I finally got some time to work on my amp. I have removed all the old parts ( which I plan on reusing, some) and got to my OT. After applying about 12V AC to the primary, I got 1V AC on the secondary. That gives me 12 winding ratio. Not much. If I use 2 push-pull 6L6 output tubes and therefore double the output impedance, I get, on average, 4600 ohms primary impedance. Is it enough for a pair of 6L6? I heard they recommend 6600 ohms...