Looking for information to restore a B410

[(natey)]

Greetings, all!

I recently became the proud new owner of a 1974 (I think) B410. It's definitely a good news / bad news situation. The good news is, I got it cheap! The bad news is, it's missing a few things...like output tubes. And the reverb tank. And the cabinet. And I'm pretty sure the output transformer is blown.

Of course, this was expected, since I bought it as a project, and I'm pretty confident in my ability to electrically restore this amp. I've got the schematic, but there are a few pieces of information that I need to really get the amp working to original spec again, and the googler has been unhelpful on these points:

1. What are the specs of the output transformer? I know it's a 4 Ohm output, what about the input? It looks to me like the B410 output section is identical to a fender twin reverb - can I just use a modern replacement designed for the twin, or are there other factors to consider?

2. What are the specs for the original reverb tank? I don't know what model tank came in the amp originally, and so I don't know which Accutronics replacement to order. (Sadly, the Accutronics replacement will probably not be made by beautiful girls in Milton Wisconsin...)


Thanks for any help you can offer...

[(soundmasterg)]

Greetings, all!

I recently became the proud new owner of a 1974 (I think) B410. It's definitely a good news / bad news situation. The good news is, I got it cheap! The bad news is, it's missing a few things...like output tubes. And the reverb tank. And the cabinet. And I'm pretty sure the output transformer is blown.

Of course, this was expected, since I bought it as a project, and I'm pretty confident in my ability to electrically restore this amp. I've got the schematic, but there are a few pieces of information that I need to really get the amp working to original spec again, and the googler has been unhelpful on these points:

1. What are the specs of the output transformer? I know it's a 4 Ohm output, what about the input? It looks to me like the B410 output section is identical to a fender twin reverb - can I just use a modern replacement designed for the twin, or are there other factors to consider?

2. What are the specs for the original reverb tank? I don't know what model tank came in the amp originally, and so I don't know which Accutronics replacement to order. (Sadly, the Accutronics replacement will probably not be made by beautiful girls in Milton Wisconsin...)


Thanks for any help you can offer...

Are you sure the output transformer is blown? They don't often fail, though many people think that they have when the problem is something else. What steps have you taken to know that the OT is bad?

That whole circuit is very similar to a Twin Reverb....not just the power amp.....you could likely use a Twin Reverb OT though I do not know that for sure. Schematics do not usually list OT specs. The reverb in those amps wasn't as good as the Fender. I would suspect they likely set up the circuit impedances very similar to what Fender did, and if that is true, then you could use the typical Fender replacement tank and get a better reverb sound than stock. The MOD tanks that AES/CE Distribution sells sound much better than the Accutronics or Belton tanks these days and are cheaper too.

Greg

P.S. I know someone locally who might be selling a 1974 B410 soon that is in perfect operating shape aside from some dirty pots and a likely electrolytic cap replacement that should be done throughout the whole amp. I played the amp and it sounds great aside from the problems I mentioned.

[(natey)]

Hi Greg, thanks for the response!

For the output transformer, my ohmmeter reads infinite resistance between one of the two input side leads and the input side center tap. It also reads a very low resistance between the input side center tap and the output side leads, which probably accounts for the fact that the fuse blows when I switch the amp from 'standby' to 'on.' I understand the OT was originally a well-built part, but from the corrosion and dirt on the chassis, I wouldn't be surprised if this amp had been in a flooded basement at some point.

I will definitely check out the MOD reverb tanks, but I don't think I can use the same impedance specs as the twin reverb - fender drove their tank through a transformer, and the B410 drives the tank straight from the driver tube. If you could find out from your friend selling the B410 what model number is on the OC reverb tank, that would be a great place for me to start looking for the specs of the original tank...

[(natey)]

Update: definitely a bad output transformer - I pulled it from the circuit / chassis and verified both an open circuit on the input side and a short between input and output side. Mojotone makes a twin reverb OT that's the right physical dimensions, so I'm going to give that a try.

I'd still like to find the specs on the original reverb tank. I've compared the b410 circuit with the twin reverb and they're different enough (different driver tube, transformer on the input side, different bias voltage on the recovery tube) that it's not really any help.

Anyone know of an amp that uses the same reverb scheme as the b410?

[(natey)]

OK, not much response, I see...I will continue to post updates anyway, in an effort to help out the next person that tries this.

I bought a Mojotone 775 output transformer, which was apparently designed for the blackface Twin Reverb. The mounting flanges are the wrong dimensions entirely for the B410, so the 775 can't be mounted vertically without substantial help. However, if you remove one of the side plates of the 775, it is exactly the right dimensions for mounting horizontally (the same way the original B410 OT was mounted).

Then the problem becomes the fact that the leads from the primary and secondary come out opposite sides of the 775, so one set of leads will be outside the B410 chassis. I fixed this by drilling a new hole through the chassis near the OT, installing a rubber grommet, and threading the wires through it and back inside the chassis. Not difficult, but something to be aware of.

I can't compare to the original OT, but the 775 sounds good to my ears - very detailed, and plenty loud. Time will tell, but so far the 775 seems like a good option for the B410.

Next up: I have a MOD reverb tank on order. I'll post another update when I've had a chance to try it out...

[(jps)]

OK, not much response, I see...I will continue to post updates anyway, in an effort to help out the next person that tries this....

This is probably due to not many having this amplifier in the first place.

[(soundmasterg)]

Sorry for the delay...I hadn't been on the site here since I made that previous post except to check some quick info......I finally got this other B410 from my friend in for a refresh and it has the original reverb tank. Looking at the reverb portion of the circuit, the Twin Reverb setup is very different from the B410 for sure....using a 12AT7 in the Fender vs a 12AU7 in the B410....a transformer in the Fender vs wired directly in the B410.

What would you like to know about the original tank? I'll likely have the amp here for the next couple weeks as I need to order some parts before I do the work.

Greg

[(jps)]

My Swart STR-Tremolo is sort of based on old Gibson amps; it has no reverb transformer but it uses a 12DW7 for the reverb circuit. Perhaps the B410 is an amalgamation of Fender and Gibson?

[(natey)]

Greg - If you have any way to directly measure the input and output impedance of the original reverb tank, that would be great. If not, then can you read the model number stamped on the side of the tank? If I knew that, then maybe I could find a data sheet for the tank with the original specs...

JPS - Not sure what influenced the design of the b410's reverb circuit, but it uses both triodes of a 12AT7 in parallel to directly drive the tank, so the Gibson design is going to have a different driver impedance.

I calculated the driver impedance of the b410, and got something in the neighborhood of 3k Ohms. The highest impedance Accutronics-style tank I found was 2k. I ordered one, figuring it couldn't hurt to try it. I'll post results when I've had a chance to try it out.

[(soundmasterg)]

Greg - If you have any way to directly measure the input and output impedance of the original reverb tank, that would be great. If not, then can you read the model number stamped on the side of the tank? If I knew that, then maybe I could find a data sheet for the tank with the original specs...

JPS - Not sure what influenced the design of the b410's reverb circuit, but it uses both triodes of a 12AT7 in parallel to directly drive the tank, so the Gibson design is going to have a different driver impedance.

I calculated the driver impedance of the b410, and got something in the neighborhood of 3k Ohms. The highest impedance Accutronics-style tank I found was 2k. I ordered one, figuring it couldn't hurt to try it. I'll post results when I've had a chance to try it out.

I'll see what I can get for you natey. I'll post it here when I have something.

I think you meant that the reverb circuit uses both triodes of a 12AU7 in parallel rather than a 12AT7? Take a look at the schematic and it shows that V4 is a 12AU7.

Greg

[(soundmasterg)]

hi natey,

I have the resistance of the tank, but not impedance as I don't have a way to check that. The input and output resistance is about 175 ohms. I'll post the pictures here, (reduced in size) but if you want the full size pictures then send me a PM with your email address and I can email them to you. This tank measures about 10" long by about 3 1/4" wide.

In looking at the tank I think it is unlikely that you will be able to find another. It may be easier to convert the circuit over to a Fender design and use a MOD replacement tank from Antique Electronics Supply (better than Accutronics these days). The circuit would certainly sound better if you did this I am sure as the stock reverb is not the greatest.

Greg

[(soundmasterg)]

I haven't compared the circuit here in a bunch of detail, but it is similar in some respects to what Rickenbacker did in the B410. It pay be possible to use some of his suggestions here to modify things to work with a common tank.

Greg

http://www.valvewizard.co.uk/reverbdriver.html

[(AndyM)]

When I was fixing and modifying amps, I came across a tank identical to the one pictured. It was in an old Peavey solid state amp. You might consider looking on Peavey forums or user groups or look for someone selling an old amp for just a few dollars. The one I found came from a Peavey mixer/PA.

[(natey)]

OK! After many months, I finally got a chance to get the B410 back on the bench and experiment with reverb. Based on Soundmasterg's info, and my own impedance calculations, I ordered a MOD tank, plugged it in, and gave it a shot. As you might expect, it didn't /quite/ work the first time...

Fortunately, it wasn't because the impedance was off! It turns out the tank I'd ordered had a different grounding scheme than the original. I saw on the schematic that the 'receive' end of the tank had a ground connection, and the 'send' end didn't. So I'd ordered a tank with the output connector ground shorted to the tank, and the input connector ground isolated. But, it turns out that the input side ALSO needs to be shorted to the tank, because that's where the input signal ground comes from.

Fortunately, MOD tanks are extremely easy to change the grounding scheme on - I just put a solder blob between two adjacent pads, and it was done. And Eureka! We have reverb, and plenty of it.

For those that follow, the Accutronics / MOD part number for the tank I'm using (with the corrected grounding scheme) is:

8FB3A1B

Note that this is for mounting on the floor of the cabinet...since mine didn't come with the cab, that's where I've decided to put mine. If yours has the reverb tank mounted in a different orientation, adjust accordingly.

Next up: putting the tolex on the custom-made replacement cab. I'll post pics when it's done...

Last thought - any recommendations on shielding for the inside of the cab, around the chassis or reverb tank?