Bedrock 1200 - Circuit discussion

[LooseChange]

I have a Bedrock 1200 on the bench. Supposedly it was rewired to be a bass amp and the owner wants me to put it back to stock.  Please take a look at the schematic below and help me understand the signal chain.

If I am looking at this right, this is what I see:
Input -> Gain -> Volume -> loop -> Gain stage -> cathode follower -> Tone -> PI -> etc.
And the reverb: From the Volume -> Gain stage -> Parallel tube (reverb driver) -> Reverb pan -> Reverb recovery -> Reverb volume -> Mixed back in at the loop.

Seems like a lot of gain is spent on driving the reverb and really not much gain on the dry signal.

Just want to be sure I'm not missing anything. Make sense??
Thanks!
http://bedrock27.tripod.com/sitebuildercontent/sitebuilderpictures/1200schematic.jpg

[Willabe]

Hey LooseChange,

Im getting a blank screene, no schemo.   

   Brad

[LooseChange]

Try this...

[Willabe]

LooseChange,

Thats got it.   


     Thanks,   Brad   

[LooseChange]

Seems dumb. All that gain wasted on reverb!
This amp layout is terrible. As I work though it, I keep on thinking this thing is going to be noisy and oscilate like crazy.
Here is a link to some pictures of the 1200.

http://bedrock27.tripod.com/id1.html


Thanks!

[Willabe]

Man, that is one heck of a balancing act going on between the dry/verb! V1b to V2a, why did they wire it up like that? I wonder what they think they gained doing it like that, you'ld have to hear it/play through it to know. Seems alkward to me, as you turn up the vol. you are also turning up the verb send/feed. They are tied together, then you go and adjust the verb. control? Like you said this is giving up gain to the dry signal path to give it to the wet. Wonder what the values are for the series Rs in there? I guess if it sounds -- much -- better or has some kind of refined quality or texture to the sound ok, but it seems they might have gone to a lot of trouble to "reinvent the wheel",  other wise it ends up being different to be different? PRR has said it before "Fender was no fool". The proof will be when you hear it, one way or the other. LooseChange you've got a good set of ears on you and have played through and built many different amps, like to know what you hear/feel after you set it back to stock.

OTOH, hey what do I know, maybe it sounds real good?


     Thanks,   Brad    


   "Just came in to join a crowd"   Tom Waits        

[tubenit]

Seems dumb. All that gain wasted on reverb

!

That is an odd design, IMO. Not sure why they did that.

IF you had smaller wires running in the layout, it would look alot neater possibly.

I remember from yrs and yrs ago, there was a tube boutique shootout ......... & Bedrock amps  were in that and got a reasonable review tone wise.

With respect, Tubenit

[LooseChange]

I was able to get everything installed and working.  Bigger challenge than I thought.  Take someone else's idea of a layout and reassemble without a map. To make things worse, the values on the schematic were tough to read if legible at all.

It is a very good sounding amp even with all those green caps (Korean). The reverb is not over the top and works well. A bit generic. It needs a bit of tweaking but I'm trying to stay stock.

>>> There is one component value I just can't figure out.  The "shift" cap at the tone stack. Anyone?

Thanks!

[PRR]

Without part-values, I can only point out that it looks like a single-input 5F6-A, with reverb added. Should be the same signal gain as 5F6-A, and that's OK for some people.

Reverb always needs three tubes: gain, power, and recovery gain. Many Fender Reverbs hide this by tapping higher along the signal path and re-inserting at an added stage which has the 3Meg:100K divider in the dry path.

[LooseChange]

IF you had smaller wires running in the layout, it would look alot neater possibly.

Yeah, I'm not a fan of the fat wire either but certainly not going to change them all...  

[LooseChange]

Still looking for the cap value for the tone stack shift switch. (say that five times fast).

[VMS]

Still looking for the cap value for the tone stack shift switch. (say that five times fast).

I suspect that there is 250pF (C24) cap in series with 47nF (or most likely 4,7nF) cap.

K1 - relay

I say 4,7nF because BC75 has the same pull shift:

http://bedrock27.tripod.com/bedrockamp/id4.html

[LooseChange]

Aha!  I finally get it...  The shift IS K1. It just bypassed the 250p and allow the 4k7 to work on it's own.
What threw me off was the K1 symbol is drawn like a cap.

So with the 250p and the 4k7 in series it's like ~235p and the shift (bypassed 250p) is 4k7.

[Willabe]

Hi all,

Dooh, I realy got that wrong about "the balancing act" on the verb,     but I think I see it now. The vol. on the amps are doing the same thing, there just wired up in different places along the signal path, Fender vol. is pre gain stage (hanging on the grid) and the Bedrock vol. is post gain stage (off the plate coupling cap).

I think your right (fwiw) the Bedrock is giving up some gain potential like you said LC, but I think that this is on purpose tho. PRR said its hard to know what the gain works out to be with out the parts values, but it maybe about the same as a 5F6-A, and if PRRs right (when isn't he?) it got me to thinkin humm.....  I think this might be Bedrocks version of a 5F6-A Bassman/50w. plexi, but with verb. They did not want to add/change the gain/tone of the preamp, just add verb. to the amp and this could be the answer, as to how/why they choose to wire it up like they did. Again maybe thats why they, as I now think wrongly said "went to all the trouble just to reinvent the wheel". I think I was wrong (again    ). There was a method to what I misunderstood as madness.  If you think about the companys name it fits. It sounds like they were/are into vintage amp tone hence the company name "Bedrock"(of amp tone?) , works nice.

Now you do have the option still laying on the table to change that and add the extra gain back in that they (Bedrock) left out, maybe as easy as wiring it up like a BF verb? Only 1 gain stage down the line (like PRR said) would get it done? Gets the verb and keeps the (extra?) gain for the dry side that Bedrock didin't (like/want?). Thats what I was trying to say/get at when I quoted PRR about saying "Fender was not a fool" and that Bedrocks schemo seemed "alkward" (IMHO). Leo's choice seemed more economicly implamented, his not wasting avalable gain, but that was a different pre and thats maybe not what Bedrock was after? So it was worth the price/tradoff of keeping a more "traditional" tweed/plexi tone, albeit with verb? LC should be easy for you to change it and tweak it, you've done a lot of that, but maybe it sounds real good as is? Guess it depends on what you or your client likes.
 
Will have to wait to hear what you think the amp sounds like when your done setting it back to stock.


Oh! I forgot, looks like V5 has a triod left over --unused-- to play around with! There's the BF mixer tube, if you need it. You could also "steal" the CF for even more gain(by wiring up the tone stack from the plate instead of off the cathode) I'm sure I not saying anything you don't already know. ("Just came in to join a crowd")


     Thanks,     Brad    


    "Just came in to join a crowd"   Tom Waits