Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rsr on January 13, 2018, 11:39:46 pm
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i have a blues jr that i want to rebuild and am considering using the hoffman bjr conversion kit.
The "donor" amp is a cream board PR 295 tweed from about 2005, as far as i can tell. It has the billm tonestack and associated mods, upgraded TO20 transformer, and standby switch.
When this amp is working right, it sounds absolutely amazing. But, it has some intermittent issues that i believe are more related to physical issues than any of the actual components or circuit design.
Hence, my goal is not to build a new blues jr clone from the kit, but rather to restore the one i have to proper working condition. So I'm thinking of it more like a transplant of components from the current PC boards to the hoffman turret and tube boards. I plan to mix new components from the parts list, as well as retain/reuse the filter caps and other changes from the billm kits.
I've reviewed the parts list, drawings, pictures and so forth, and have a few initial questions.
1) There's way fewer components in the hoffman board than in the amp currently. I understand that the reverb circuit is omitted and i'm fine with that (i agree with the general opinion that it doesn't sound very good anyway) Does that account for all the differences?
2) The wiring diagram in the PDF file i've seen appears to be very clear about how to connect the components from the parts list, but what i'd really like to know is how those turret board positions relate to the component numbers on the original BJR circuit. I mean, which of the turrets correspond to R1, R2, C1, etc... If i knew that, i think i could work out where the billm changes belong on the board.
Thanks in advance for some guidance.
roger
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There is no correlation between Hoffman's reference numbers and the original reference numbers.
So, in light of Sluckey's noting this (which I missed) .................... my following comments simply will help you correspond Hoffman schematic to Hoffman layout
Example: Matching the R3 from the schematic to the layout is relatively easy, it's just going to take you a little time.
V1-1 (pin 1) on the schematic is R3 with a value of 100k. Look at V1-1 (pin 1) on the layout. It's 100k, so that's R3. Start that way with corresponding parts from each pin on each tube on the schematic matching to each pin on each tube to the layout.
Once you've done that, the rest should fall in place. R1 is a 1M resistor on the input jack on the schematic. There is a 1M on the input jack on the layout, it's R1.
You are capable of figuring this out on your own.
With respect, Tubenit
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On the right side of the page, there is a list of Bill M mods including for the tone stack.
http://billmaudio.com/wp/?page_id=909
With respect, Tubenit
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So I'm thinking of it more like a transplant of components from the current PC boards to the hoffman turret and tube boards.
That may work for some components, but the leads will be too short to fit your old components on the Hoffman board. I highly recommend using all new components on the Hoffman turret board.
1) There's way fewer components in the hoffman board than in the amp currently. I understand that the reverb circuit is omitted and i'm fine with that (i agree with the general opinion that it doesn't sound very good anyway) Does that account for all the differences?
No. There are plenty more changes also. The differences are enough that you should only consider the Hoffman circuit to be a general copy of the original.
2) The wiring diagram in the PDF file i've seen appears to be very clear about how to connect the components from the parts list, but what i'd really like to know is how those turret board positions relate to the component numbers on the original BJR circuit. I mean, which of the turrets correspond to R1, R2, C1, etc... If i knew that, i think i could work out where the billm changes belong on the board.
There is no correlation between Hoffman's reference numbers and the original reference numbers. For example, original R1 is Hoffman R2. Original R2 is Hoffman R1. Original C1 is Hoffman C2. As you continue comparing the two schematics you will occasionally find a component that has the same reference number, for example R3, but they are few. You will get into trouble pretty quickly if you try to make a conversion chart. It's best to just follow the schematics and not even think about reference numbers.
I suggest you do this conversion exactly according to Hoffman's plan. After it's up and running, then tackle BM's mods one at a time. You may find that some of them are no longer needed. You may also find that some cannot be adapted to the Hoffman circuit. We can help you figure all that out.
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I suggest you do this conversion exactly according to Hoffman's plan. After it's up and running, then tackle BM's mods one at a time. You may find that some of them are no longer needed. You may also find that some cannot be adapted to the Hoffman circuit. We can help you figure all that out.
I think that's a good suggestion. Get it to a good working state, and then decide which mod(s) to reapply.
One thing for sure, working on one of those turret boards as opposed to a PC board will make experimenting with modding things go a whole lot better.
That said, unless there is some specific reason not to do so, i will probably use the billm filter caps i have in there now, rather than the ones from the hoffman parts list. I know what you're saying about the leads being too short trying to detach and reuse components, but for these i think it will work fine, so long as there is space enough for the capacitors to sit down between the turret legs. If that is the case, the leads will be plenty long enough.
The capacitors values that i have from the billm mods are
(1 piece) 100uF 450v (vs 47uF 500v)
(3 pieces) 22uF 450v (vs 22uF 500v)
These were all 500v in the original BJR circuit as well. Not sure why the billm caps are 450v, but i think that's still comfortably above where the amp runs, so should be fine, right?
There's a handful of other capacitor changes, but i'll start with the straight hoffman parts for those, as suggested.
Thanks for the advice.
roger