Is a 50v Presence cap too small for 100 watt amp

[Guy77]

I was in a brand new 100 watt 2rock bloomfield drive amp recently. I noticed that the voltage rating for the presence cap seemed small.
It was a 1uf 50 volt cap. Isn't this voltage rating too small?
I know fender uses a much bigger voltage rating here.
It was curious if anyone knew what brand these brown resistors were. They all seem to be 1/2 watt size. They are using Nichicon radial power caps 220uf.

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Guy

[mojotom]

Hello Guy !

Voltage is quite low at that point so no worries running a 50V rated cap (there is a 4k7 resistor between the output Jack where the feedback is tapped and that point with 390R and 1uF to ground).
Most Fenders didn’t use a cap and presence pot across the 47r/100r resistor. If some use a high voltage rating cap then this is more a convenience thing rather than a real necessity.

Btw is it a Reverb ODS that Bloomfield ?
Did you trace that amp ?

[Charlie Wilson]

Presence cap only sees about 1/2 DC. Those resistors look like they are Koa Speer 1/2w carbon film.
CW

[pdf64]

The VAC will almost certainly be a lot more than 0.5V, but the max peak level will depend on the circuit details.

[Guy77]

Hi guys, thanks for the quick reply's and the info on the resistors!

Hi Tom! I didn't trace it but the reverb is coming of V1 and entering V2 via that resistor going from v1 to v2. This is just like the John Mayer amp feeds its reverb except this is only using 1/2 of v2 and then the other 1/2 of v2 is also used for the reverb.

Then v3 is actually your tweed mixer. Finally v4 is your overdrive tube just like in an Overdrive Special . There are no snubbers on the overdrive tube as seen in pics. The OD entrance is a 220k resistor feeding a 100k trimmer. The overdrive sounded a little thin and the owner wants them a little fatter. I will be adjusting some of the caps on the overdrive section to fatten things up there.

Cleans sounded amazing! Fender style cleans.

This amp is running low plates ( 100K) . The v1 tube is running at 176v. I thought that was a little low for these type of amps. I more use to seeing things running at 200v on v1. Wall voltage was 121v.
Apparently its almost a 1 year wait list for these amps now.

G

[mojotom]

Thanks for the update !

So this is a pre OD Reverb, interesting !

[Guy77]

Yes I was surprised to see this pre OD reverb as well Tom. What I really like is the shielded cable they use!
This has to be the easiest shielded cable to work with. The shielded part is actually in its own sleeve. If you magnify the images I posted you will see the shielded wire is actually 2 separate white sleeves.

I would really like to find out you sells this.

G

[jabguit]

Then v3 is actually your tweed mixer.

G

Tweed mixer? I thought tweed was woven.......

cheers,

[dimitris]

Did you find out by any chance what are the values and the role of the capacitor and the trimmer on the pab relay board?

[Guy77]

Did you find out by any chance what are the values and the role of the capacitor and the trimmer on the pab relay board?

Hi

I will check the cap values on my next visit to the amp. I believe the cap and trimmer are part of what's called a "treble bleed" circuit for the Overdrive channel.

Cheers
Guy

[rootz]

That's a nice amp on your bench Guy!

Looks like a low plate classic to me, but with some tweaks. Definitely a pre OD reverb like #13, but with less preamp tubes. Is there a tweed mixer in there? I don't think there is. Looks like the reverberated signal is mixed with the dry signal at the relay and with just a 270k resistor (on the main board after a small coupling cap).

As far as I can see there are no snubbers on the OD triodes. Coupling cap from V1 looks small, probably 10n or 22n max. Interesting cap board. The psu cap for the reverb looks to be the first next to the caps for the plates. I suspect the small cap next to it is for the screens, then pi, od, clean stages. But then where does the B+ for the mixer stage come from? It would not surprise me at all if all reverb related stages are tied to that 27k resistor and cap on the psu board. The rest of the psu looks like what to expect: 2k2, 22k, 2k2.

The cap and trimmer are on the PAB board. Is it a hf bleed only when PAB is on? I can't see a wire to the OD level pot. I could be a hf bleeder early in the preamp (tone stack). Definitely something to trace.

I'd love to see more of this amp. I'd be happy to trace it, but it's quite a journey from here to Canada

[talbany]

I don't think it's a "Tweed" either. The tweed utilizes 2 12Ax7's for the mixer section, one for dry and one for wet side mixed at each cathode.

Tony

[rootz]

Right Tony and you'd need some extra parts that are not on the board. I think one of the 'neat' tricks is that they ditched the tweed mixer and went with simple passive mixing. I don't really understand how the mixing is done, though there are only so many ways to do this.

I attached a schematic I drew a long time ago based on just one gunshot of a Bloomfield I could find online. I wonder how close I got without actually ever being in such an amp. Biggest points of interest: PAB relay with cap and trimmer, OD relay wiring, pot values and taper, the small resistor (10meg?) on the 4th preamp socket (why is it there?) and voltages (I guess rather low, around 180V on V1 plates).

Oh and when you're at it Guy , are all preamp tubes 12ax7's?

[rootz]

Oh and about that presence cap. 50V is plenty! Dumble got away with reversed tantalum caps even. There is very little DC over the cap and AC swings less than a tenth of the actual swings at the output jacks.

[Guy77]

Hi everyone! Thanks for all the input.
Hi rootz! The amp is all 12ax7 except for the one reverb driver tube at v2 , its a 12at7. I will check OD section pot values .

Your right Tony I miss spoke when I called it a Tweed mixer.
The mixing is done at v3 using a similar setup as the John Mayer Signature amp except the JM has this mixer at v4.
Coupling cap from v1b is 22n.
The cleans on this amp are very nice! So #13 would probably have beautiful cleans too. Maybe the way the reverb feeds off of v1b effects the cleans in a positive way. I know the JM feeds the reverb in a similar way and it has great cleans too.

I will take more pics when visit again in a week. I attached the manual here showing the function of each preamp tube.
I believe the reverb signal is going from v3 (mixer)to blue cap on board and then through a resistor ( looks like a 270k resistor) and from there to a relay board. The clean side of v3 hits the orange cap on the board with no resistor after it.

I will take a closer look at your schematic when i get a chance rootz, thanks!

Bloomfield-drive-manual.pdf

Cheers

Guy