ODS build from SF Bassman 70

[FUCHSAUDIO]

Some Bassman-70's had the wacky Marshall style HV secondary that required the CT to be connected between the first two main filters and used a bridge rectifier. Make sure (before you dismantle it) that you know whether you have this configuration. If you light it up with a standard supply, you will have very low B+. I believe "normal" B+ is around 500-ish...add a choke and get about 550 (stock has no choke).

No harm, but it could happen. Some had separate bias windings as well. FWIW, the Bassman-70 is a nice D-style platform. I've done quite a few, and they came out great !

[guitardude57]

With the digital age, I think that the Printshops out there, make screens/plates easy.

They have used negative plates for years, for raised ink printing

Just have to find one that will do it for a good price.

Trophy makers is also an option.....etching. Don't care for that personally.

[guitardude57]

Thanx for the info Fuchsaudio.

I have Thursday and Friday off, so I will open her up and see what we have in there. I'll take a few pix for documentation on my build as it goes.

I'm sure I will have some questions on things to look for, and I know you have built great amps one could hope to aspire to. This amp is for me, and want it to be the tone. Thanks again. Mike

[FUCHSAUDIO]

Thanx for the info Fuchsaudio.
This amp is for me, and want it to be the tone. Thanks again. Mike

Don't we all ? lol

[guitardude57]

Yepper, we sure do

I looked at the schemo and see what you mentioned about running CT to Filter. Interesting. What is the purpose of that?

I have also heard that folks also disconnect the UL taps as well. What is your experience with this ?

Also, I have tried to find the ODS 50 Rev E Docs with tweaks, BOM etc with no luck. According to BobW this is the more current Rev.

Mike

[FUCHSAUDIO]

It's like the benefit of a bridge rectifier, and the CT connection splits the voltage equally across the caps. A choke gives a nice lift the B+ too. Don't worry that you're up in the mid 500's, adjust your dropping string to the preamp accordingly, and bias to 36 ma per power tube, and it should come out swell. Healthy 60 watts with a choke. It never was 70 anyway .

Just tie off the UL taps, and the output transformer should be fine. It's 4/2 ohms as I recall, but some had an 8 ohm winding tied up and not used too. Unless you have something laying around that's a better one, and non UL already, it's not that big a liability imho, unless the output impedance is an issue for you.

[guitardude57]

OK that makes sense.

Which caps/and resistors do you recommend for the preamp board? All OD's or a mix.......? I am not opposed to using slightly larger resistors if better in any part of the circuit.

Any BOM's found, specify particular components, but with your insight.... you may have better ideas..........

I do have a generic Weber eyelet board ( filter cap spots on one end.....3 row eyes across the rest) laying around.
I have a board layout that could be transposed onto G-10 if need be.

Thoughts?

I want to start ordering parts on Friday.

Once everything is done, tweaks and all, I will compile everything for a sticky.
Will make it a little easier for others to find this conversion in one place, to do the same if desired.

Thanks, Mike

[talbany]

Mike

FWIW...Bassman 70's transformers P/OPT are rated closer to 100 than 50( for linearity)..So they can handle either 6L's or 6550's/KT-88's.. @ 500v.. (personally I prefer the tone of the 40 watt tubes being run at 500v than with most 6L's)..Anyway something to try since you can run either or.. Just don't forget to re-bias between sets..Tie off the taps like Andy suggests and after you get it built try a reverse speaker cable as in my experience can be a noticeable increase in bass response bloom in these UL OT Fender used in that era..(Maybe due to the opposite polarity of the JBL's.. Don't know can't prove it) it just works... Good Luck..

Tony

[FUCHSAUDIO]

Which caps/and resistors do you recommend for the preamp board? All OD's or a mix.......? Any BOM's found, specify particular components, but with your insight.... you may have better ideas.. ........Thanks, Mike

I'm sure the forumites can guide you on this. I have my preferences, but there's a wealth of data on TAG as far as passive part types, values, voltages etc.. I don't built what could be considered "a clone", and thus the part types and values I use wouldn't really apply to your build.

[guitardude57]

I just snooped around Mouser's site,
and put together a parts cart from the:

ODS BOM_120_418_146 80's and 90's style found in the files pages.

All the pots were marked obsolete, 80% of the rest of the parts were either out of stock, a 300 piece minimum order, or "no match" for the listed part numbers...........and their part numbers listed on the BOM......................bummer.

What's up with this? Any options short of having to order just resistors and caps from gobs of suppliers?

I need pots, resistors, all the caps. Most resistors and wire is easy to get near me here. My regular supply house is lousy for amp value caps and pots. Boy, they sure have mass quantities of oddball stuff. Need a 600V .1 ........no gots.

[guitardude57]

OK, ordered all my parts that my regular supply didn't have.
Just need to get wire, relays, 12V tranny and power supply caps.

How much wire do you guys usually use to build one of these?
I guess I am going ahead with the HRM, possibly with a switch out of circuit if wanted. Open to ideas...................

I am using the Bassman 70 OT and either the Bassman PT or an old Wurlitzer 375-0-375 with mega taps out the wazoo.......been wanting to use this for something. it does have a 12V tap, 5V, a couple 6.3V, a 50V etc. Damn thing powered 42 tubes, and is in great shape.

Opinions, Andy?
Thanx, Mike

[Sonny ReVerb]

How much wire do you guys usually use to build one of these?

Check out Apex Jr. for you wire needs. It's a good idea to call him to see what he has on hand. If you get 10' of five different colors you'd have plenty. I'd probably go with 20 gauge stranded with 600V or better insulation. Teflon is nice is you're just getting comfortable with the soldering iron. Solid wire is fine, but you can get hidden breaks if you bend it too often. Use the 'search' feature and you'll find plenty of guidance on wire.

[guitardude57]

Thank you, I will check out Apex. going with solid core pvc 20. Teflon isn't worth the expense.

Solder and I are good buddies. 40 years worth (thru-hole), and down to 5 mil spec (SM) under the microscope stuff when in the corporate world.

Which of these amps have you built?

Mike

[Sonny ReVerb]

Which of these amps have you built?

I haven't built a D-style amp yet (unemployment sucks), but I have built several other amps.

I see you're in Indiana. I'm just south of Indianapolis.

[guitardude57]

Well, I have most of my passive parts now, (enough for a couple builds) and trying to decide which of the "D"'s to build.

I got a set of jborders nice HRM boards, and have been studying the various layouts around here.

I am inclined to do the 97 ODS layout, or the Bassman d final layout 186 with the tube FX loop. Now to get a preamp board put together to do this. Just enough differences to make it interesting.

Does anyone have either of these layouts (or similar) ironed out with good wiring layout, and matching schemos?

Once I get a good main board to duplicate, I can get drilling tomorrow.

Thanks, Mike