Bassman 70 into ODS

[guitardude57]

Howdy,

I am picking up an early 70's Bassman 70 for reasonable money today. A good candidate for an ODS. I have seen somewhere about Norm doing a conversion, and can't find the info, BOM etc.

I more than likely will do a full gut-out. Want the full relay and switch layout, and if there is space, will include the dumbleator circuit as well.

Any ideas/comments/circuit suggestions welcome.

I am not concerned with the value of the original amp. I am getting it specifically for a Dumble knockoff.

Thanx, Mike

[Structo]

So I take it you are going to build a 50w version?

I see that the PT has a bias tap, that is nice.

Do you know how you will power your relays yet?
Separate filament xfmr?

[talbany]

I've not seen a Hybrid layout W/D-lator built in..Posted yet. Here is an HRM layout you can modify (Axe the HRM tone stack pot values etc..) as it's the same size... A few tips

You can still keep the cap can dog house and add the clean channel V1 cap inside down by V1 like Dumble if you need....This will give you plenty of room to add the Dlator/ relay power supply transformer etc inside.. I would also Axe the UL taps on the transformer..Good Luck!!

Tony

[guitardude57]

Hi,

As I have just started looking over some schemos and whatnot, on the ODS......... not sure how to power the relays yet. I just picked up the B-70 and got home a few minutes ago. I have Friday off, so I will then take a good look inside. The amp is in real nice shape and well taken care of. I am really excited about this.

Thanks for the info. This being my first ODS, I'm sure I will have questions as I go.

[PrecisionB]

If you run your heater voltage (6.3 Vac) into a rectifier you will get approx. 8.91Vdc (remember 6.3 is a RMS voltage value, not peak) From there you can easily run 9 volt relays. Just make sure your heater tap on you transformer can handle the extra current (which it most likely can) No extra transformers required!

[guitardude57]

I thought about a recto on the heaters. I am using the '77 Bassman 70 trannies. The PT does have that weird CT to the power filter junction set up with Diode bridge. I do have an old Wurlitzer organ PT that powered 42 tubes....... that would work, but it is a 375-0-375........ a little much.

I am ordering the board set from jborders5 tomorrow, so I will see better how to tackle that issue once they arrive.

Then get the relays, with board in hand to get the right ones. MAI has several with different sizes and pin layouts.

I ordered all my caps, (but the power filters) pots, and choke from Weber.

9 Henry choke....should be OK?

Got all my resistors from my local supplier, MAI.

I have the next 2 days off, so will pick up my wire at Meunier Electronics in Indy. They should have the 100uF 350V's and the Dales there too.

Once everything arrives, I have all I need to get going.

Suggestions/comments welcome and appreciated. Mike

[Structo]

The filter choke on the later ODS amps is the Fender 022699 4 mh 90ma.

AES is having a 10% off everything sale right now.

http://www.tubesandmore.com/

[PrecisionB]

careful, those weber pots are absolute junk!

[guitardude57]

Got the amp finished, minus relay board.......hard wire switched. My relay board was just too big to use.

See the PDF to see basic layout.

Only issue is, I need to drop the voltage string to get all my voltages down some.

Main B+ is 516 Volts

B+5 is high....... 430 volts feeding plates so mid 200's on the pins V1

Everything works function-wise. Bias pot gets me to 43mv no red. Need to change the tail resistor. Have a 33k there now. Range on pot is about 78 to 43mv. Would like to shift it down 10 or so. What do you think?

The clean channel has some hair, may change to low plates on V1 to make it a little cleaner if the voltage drop doesn't do it. Needs a little sparkle, no bright cap........high voltages...........

Switches/pots in front:
Fat/Thin
PAB
V-T-M-B
Overdrive on/off
Drive
Level
Master
Tube FX loop in/out
Presence

Damn near dead quiet, no hum.... very little hiss
Set OD trim at 33k
Big OD.

Played it a little to check functions, keeping an eye on the things. It is very close. Never built an internal tube FX loop before. What a difference just running it switched in! Wow! Once the voltages are right, can't wait to plug in the rack.

Pretty happy that it worked without pulling the last of my hair............

Will have pix soon, need to get new batteries for the camera.........

Comments/suggestions always welcome. Mike

[Structo]

I would get those voltages down where they should be before tweaking any values.
You will be surprised at how much the voltage affects the tone.

Then try different tubes in V1 and V2 when you get the voltages close.

You can get up to a 15v swing from different tubes.

If you installed a balance trimmer on the PI adjust that for maximum bloom into harmonic bliss.

But I wouldn't do that until you get your preamp sorted.

Good luck!

[Structo]

I was looking at your layout.
You should have the hot wire go to the fuse first.

I don't understand that wire from the fuse to the 500R 5W resistor that goes to the sceen filter?

[guitardude57]

Yep, borrowed layout............. Fuse is before switch on mine, like on all my builds. I looked at the schemo included on this post, and the tweaked layout, for ideas with and without relays. When I get the relay board situation handled, I'll just rewire the switches.

Silly when you see some schemos or layouts and they fuse after the switch, or worse still........... on the neutral. I think the idea on that one is to not get a zap screwing in the new fuse. Silly Rabbit, unplug the amp when replacing fuse....... no tri(ck)-(x) there.

Anywho......this morning before work I touched up a few eyelets and such.
Got the bias voltage to get down to 32. So, we have a 30 volt sweep on the pot. Good. Tonite start attacking the dropping string to get the voltages in line.

As is now: at 119VAC

Dropping string:
choke-4k7-12k-10-150k fet sim

high plate values on V1 and 2
220k/3k3/5uf 180k/2k2/5uf Second V2 Ck 1 uf


B+ 497
B+1 497
B+2 495
B+3 422
B+4 355
B+5 315

V1 1 168 lowered since reflow but obviously too low
V1 3 3.22
V1 6 108 way low
V1 8 2.09

V2 1 193
V2 3 1.65
V2 6 242
V2 8 2.03

V3 FX Loop Not sure what voltages optimum here. 1 and 6 300+volts

V4 PI
1 307
3 54.4
6 302
8 54.4

All grounds read good. 4 separate groundings, nice and quiet.
Weird voltages.

Structo, whatcha think?

Mike

[guitardude57]

Structo,

The layout uses the ground selector switch as a Choke/Resistor selector.
This powers off the SB switch. The filters charge before stand by.

The other wire from the fuse is to the 6-0-6 transformer. I know it looks strange and won't do it that way. The fuse is on the neutral line. So one primary is hot (mains) and other neutral (fuse).

For now my little tranny is not hooked to anything until I put the relay PS and relay mount boards together.

I used this layout mainly for preamp board layout and general ideas of the construction.

My preamp board is made of 2 pieces of formica glued back to back with slightly large eyelets. Looks interesting and rock solid.

This layout made for a decent template to drill holes.

The amp functions work, is quiet...no hum. It doesn't sound bad at all, working on the voltages before tube and PI adjust tweaks. I think it will be a great sounding amp.

[guitardude57]

Hi all,

This morning reflowed a few more places, checked voltages, and swapped tubes around.

My voltages were low on V1 due to me inadvertently placing a 12AT7 in there. I have a decent selection of nothing fancy, old pulls....old Wurly RCA, Sovtek, Rubys, GE, etc. Put a GE that was non-microphonic, voltages 190 and 210......good enough for break in. Rest of preamp mid to high 200's.

FX loop tube is still low-mid 300's ............ too high.

Anyone know target voltage on FX tube? If I remember the D-lator runs low 200-220V. May need to drop voltage somehow on that B+ feed. Tried different tubes, not much change in voltage there.

Now the gain in the front end is up, the clean tone is wunnerful. Plenty bright now with higher volts on tone stack half of tube.
I'll let it break in some before I decide to swap to low plate values on V1. The little bit o hair seems OK for the moment.

The hiss factor is up on OD. Have OD trimmer at 33k ohms. The crunch needs a little sweetening, slightly harsh, slightly buzzy. Will tweak on PI trimmer tonite.

Comments welcome, Mike

[Structo]

Generally the consensus is to have around 190v-200v on V1, 200v-210v on V2 and 290-300v on the PI.

On the loop tube I'm not sure about the internal one but on my stand alone D'lator the key thing I was told was to have the cathode follower cathode at 30v.
The original accepted schematic shows 256v on pin 1 and around 210-220v on pin 6.

Also I had a problem with it killing the highs too much.
Probably due to the coax I used so I reduced the 220k to 47k to help restore the loss.