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

[Guy77]

Guy, just on more pictures request: please the full chassis in just one shot. So like you did the last two pictures, but all in one shot. The complete chassis has to be visible.

Hi Bas, I attached the picture showing the whole chassis from the top in one shot. Hope this is ok.
Please note I have changed the last resistor in the power supply board to increase the voltage on V1 as per customer request, but for people building this the first time I would leave the power section resistors at their stock values, just as listed in your schematic. Notice the pilot where the heaters wires connect, the front looks like the old school fender large jewel pilot we all know but the back where the wires attach is a newer design that does not protrude as much into the chassis. Need to try these.

<Quote>
"This amp is also not pre od reverb like #13 we all know. That one has the complete reverb section mixed in before the od relay. Here the reverb is taken from the cleans, but mixed back after the od relay. So, to me, this looks like a a crossover between pre and post od reverb."
<Quote>

Thanks Bas. This would explain why the reverb still sounds good when mixing with the Overdrive channel.

Here is what they wrote about the reverb on this 2021 model amp.

"The Bloomfield Drive features a new sophisticated all tube spring reverb circuit specifically designed to not interfere with the overdrive channel and still retain clear note definition, a sweeter top end and maintain touch sensitivity when you push the gain."

Regarding the PAB 1M trimmer it was set from factory exactly as seen in this top picture I just attached. It is set to half way so about 500K. I noticed at this setting when switching to PAB in clean mode the amp gets a little louder as expected but also a little brighter. This was with Bass pot at about 2pm on the dial and Treble and Mid about half way


Guy

Top - Copy.jpg

[talbany]

This would explain why the reverb still sounds good when mixing with the Overdrive channel!

In a way it does However the only drawback I can see with doing it this way is that when you switch from clean to overdrive you change the gain structure of the dry signal and therefore changing the amount of reverb signal mixed back,thus making it a little trickier to balance the 2 channels?..So the clean or overdrive signals might become slightly unbalanced. Do you notice any shift in the mix Guy?

Curious to know?
Thanks!

Tony

[Guy77]

This would explain why the reverb still sounds good when mixing with the Overdrive channel!

In a way it does However the only drawback I can see with doing it this way is that when you switch from clean to overdrive you change the gain structure of the dry signal and therefore changing the amount of reverb signal mixed back,thus making it a little trickier to balance the 2 channels?..So the clean or overdrive signals might become slightly unbalanced. Do you notice any shift in the mix Guy?

Curious to know?
Thanks!

Tony

I just checked this and yes that is correct Tony the amount of reverb decreases when going from Clean to Overdrive channel. I would say its about a 50% drop in reverb when switching channel. So you still hear it but not as much. You need to have your reverb at about 9am on the dial to hear it on both clean and overdrive channels.
The customer actually really likes this feature in the amp.
For me the big seller with this amp is the clean sound, it has a sweeter clean sound than the other types of Overdrive Reverb amps I have built (ex is the Reverb completely after the Overdrive) .

I also noticed the v4 overdrive tube in this amp runs at 250v while v1 is 180v. So in the schematic v4 should be getting its B+ higher up in the chain. Other voltages look good
Guy

[Guy77]

sorry 2x post

[talbany]

That makes sense to me since the Overdrive channel's signal is stronger..I think the reason why everyone likes the Clean channel is the added 1/2 stage of gain the parallel reverb provides. It gives the clean more life and softens the top end a little. That along with the NFB on V1B So if you are one that likes a little less reverb when in OD then this is the way to go
BTW for someone who want so cure this you can just set up another relay (running off the same supply as the OD/Clean) and add another mixing resistor when in OD
Thanks for checking
Tony

[norburybrook]

That makes sense to me since the Overdrive channel's signal is stronger..I think the reason why everyone likes the Clean channel is the added 1/2 stage of gain the parallel reverb provides. It gives the clean more life and softens the top end a little. That along with the NFB on V1B So if you are one that likes a little less reverb when in OD then this is the way to go
Thanks for checking
Tony

very interesting. I most ALWAYS want less reverb when in OD so this seems like a very elegant solution.

It's piqued my interest for sure.

Come on Tony, take one for the team and build one


M

[Guy77]

That makes sense to me since the Overdrive channel's signal is stronger..I think the reason why everyone likes the Clean channel is the added 1/2 stage of gain the parallel reverb provides. It gives the clean more life and softens the top end a little. That along with the NFB on V1B So if you are one that likes a little less reverb when in OD then this is the way to go
Thanks for checking
Tony

Yes that's it Tony! That is were the magic is happening.

Its that 1/2 stage of gain the parallel reverb provides!!!! I have been trying for ever to get that sound from the Reverb after the OD style amps but no matter what I did with v1 I could not get the sound of this amp!
When I build my next OD Reverb amp this will be the one !

Cheers

Happy Fathers Day everyone!

Guy

[talbany]

Yes that's it Tony! That is were the magic is happening.

Not only that but I think you also just discovered the magic behind the AB-763 circuit!!.. Ole Leo strikes again!

Come on Tony, take one for the team and build one

I am afraid the wife would finally file

Happy fathers day guys!

Tony

[rootz]

The top end of the cleans have to be a bit softer on a Bloomfield because:
1) miller capacitance of that paralleled triode loads down the high end a bit;
2) output of the last reverb triode presents a low pass (270k+2n2 to ground via output impedance/power supply);
3) the trimmer cap combo on the PAB board loads high end a bit.

Separate they don't load down much, together they will account for 3dB. Is the high end of the cleans a bit more compressed and coloured too? It was in my Custom Reverb Signature clone that had a 250k send pot with 1n cap coming from the relay. It loaded the high end a bit and sounded pretty good that way.

Tony, Guy, I don't think there is any extra gain from paralleled triodes anywhere. Care to explain how you see that? This circuit is nothing like a blackface deluxe reverb. That amp has an extra gain stage following the clean path to recover the loss of gain due to passive mixing at the input of that third stage. This amp essentially mixes at the output of the reverb and either V1b or V2b (ratio pot). This really is a variant of the twee mixer circuit in that respect, only it mixes after the coupling caps.

The different levels of reverb for clean and OD are IMHO not just explained by difference in output levels of respective stages. You can mail in the exact same volume/output levels from both. However, clean has a fixed output impedance of around 40k, the OD has a variable output impedance. So that would explain changing levels of reverb when in OD. Other than that I can think of rather different levels of harmonics in clean and OD, where the reverberated cleans just get swamped out by the more complex OD tones. Quite a different frequency response too, with more mid content than the reverberated cleans. Can I make a guess? Is the reverb in OD mode more like a soft shimmer?

I don't think the Bloomfield solution is the only and definitive answer to good reverb in both clean and OD. The Coral provided the best solution IMHO, but is arguably more complex and expensive. That amp has on DPDT relay extra to switch reverb send en return controls for clean and OD. Because of the double set of send and return pots you can differentiate the designs a bit: make the reverb for the cleans contain a bit more mids, dump a bit of high end for the OD reverb. At least, that is what I'll do in all my amps from now on.

Guy, your OD tube is wonky! With your new set V1 B+ all voltage apart from those reverb related should read more or less stock #124. Because, well, let's call a duck a duck here, this is a #124 with some tweaks and reverb added.

Interesting remark that PAB seems a bit brighter. That is what was to be expected as the trimmer/cap combo shaves of more mids than high end. Lower on the tail the trimmer shaves of more and more mids (broader band) and also shaves of a bit of extra high end.

And super thanks for the picture Guy! Is that main board 2.75" tall? Those chassis look really nicely made! And thick! Is that 3mm?

[Guy77]

Hi Bas. Thanks for all the detailed info. Yes the reverb sounds like you described in OD when set low but changes to be more like clean side when set high.Or maybe I am just perceiving it that way when set high.

I will check the board dimensions when I home tonight.
Regarding the clean sound of this amp. It sounds just like the 2rock JM Signature and I could not get any of my other Overdrive Reverb Amps to sound this way even with the same tone stack.
Thats why I am thinking v2a is adding that Special magic , softness, to the sound of the clean channel.
Guy

[rootz]

I hear you, but still it is a very different circuit than a JM signature. Surprises me it sounds like a JM signature.

[Guy77]

Here are the measurements for the boards Bas.

Power 4 3/4 in long x 3 1/2 tall
Preamp 9 in long x 3 in tall

and yes the chassis measures 3 ml thick ! The Power trans looks like the same size as a fender Twin Reverb.

Someone asked about the Reverb tank model.

I attached a pic of it below.

processed (3).jpeg

Guy

[rootz]

Wait, a Tub Amp Doctor reverb tank from Germany??

And yet another question about the dimensions Guy. How big is that chassis? 19x8x2.5"?

[Guy77]

Wait, a Tub Amp Doctor reverb tank from Germany??

And yet another question about the dimensions Guy. How big is that chassis? 19x8x2.5"?

Hi Bas. Yes chassis is exactly 19x8x2.5.

Guy

[talbany]

Or maybe I am just perceiving it that way

You are not the only one This was a problem we (VVT Amps)ran into when we were prototyping adding reverb into our Dumble style amps years ago way before any ODSR or SSS info was available. We incorporated a similar parallel type 2 tube reverb set up as the Bloomfield where we mixed it back in at the master volume, We could not keep it balanced..Now before everyone loses their minds here it did work O.K however being an amp company selling amps to the general public we thought it was not up to our standards and didn't want to deal with any complaints about not having a perfectly balanced amp w/reverb..(Perhaps this is why Dumble tried different methods of implicating reverb into the O.D.S Topology). I will however say that the best sounding better balanced method done by some of the other well known highly respected builders ( I cannot mention) sets it up Post OD gets sent through the tank and mixed back in just before the Master. (Same basic technique as Dumbleator)
Again this is not knocking Two Rock or and other Pre O.D mixing method since some people might even like the slight difference in reverb mix in O.D this would work out wonderful for you.
Now keep in mind this is what we experienced and is my own opinion agree or disagree with me, however your perception is everything regardless as to what math or modeling and my opinion tells you or anyone else..IMO

Tony