Adding tube spring reverb to a Mesa Boogie SOB

[Boogie]

I'm considering adding a spring reverb tank to a Mesa Boogie SOB. I've got a few questions...

The SOB (Son Of Boogie) was an early reissue of the Mark I, mine is from 1982.
I'm not sure if there were any SOBs made with factory reverb, however there were Mark I's, some with reverb some without, and the circuits are fairly similar though the SOB is somewhat more basic.

There are two spare 12ax7 size holes in the chassis (its a combo). There is also plenty of room in there to add things. Its a 60 watt version and they came in 100W so there is a lot of empty space in the chassis.

My main question is - what's the difference between tube reverb designs that use two tubes vs one? Most of the stuff I've found on Google forum searches are from people saying you *need* two extra tubes (actually one and a half which means in practice, two). The fender design seems to have this, with the driver tube using both halves of the tube (usually a 12AT7 it seems) in what looks like parallel on the schematic.. The second tube only needs one half of it, usually a 12AX7, for the return.

There are other designs, such as the Mark I, with only one tube for the reverb, a 12AX7, using half of it for the driver and the other half for the return. I'm pretty sure my Studio .22 is the same, with one 12AX7 for the reverb (but only a short tank which eventually I'll swap for a proper long size one).

What's the advantage of having two halves purely for the driver? Since it would be nice to only have to add one extra tube. Most people say the fender reverbs sound better than the Boogie ones, is it the extra driver half tube that improves the tone? I can choose whatever tank I like with appropriate impedance etc and I understand how the accutronics codes work for the different types of tanks.

I've also read that the reverb transformers used in early Boogies are the same as the standard Fender ones that are easily available.

Other than that, I would have to verify that the PSU can handle the extra current for the extra tube (or two). My guess is that it would since the amp is only 60W, and there are all the extra spaces to add things and presumably Mesa didn't have a huge number of different transformers for every variation of options, so I would assume there's enough balls in the PSU for at least one extra 12AX7, and maybe another? But would have to check it out in detail obviously.

I was reading about reverb pedals as an option and everything seemed to say that a real tube spring reverb is much nicer.

[pdf64]

To get a deep reverb, the tank needs sufficient input power.
Both paralleled sections of a 12AT7 can push out about 1 watt.
I've never checked but would be surprised if a single 12AX7 section could manage 1/10 watt, especially in the MB arrangement, with a big B+ dropper resistor.
The recovery really needs a 12AX7 section, though a FET eg LD150 could serve instead.
A one tube option could use a 12DW7, with the 12AU7 section driving the transformer. Or an ECL82 or ECL86; think of them as a slightly lower spec EL84 and 12AX7 section in one 9 pin tube.
There have been threads here on this option.
Reverb pedals begin to seem very attractive; reverb tank performance was always a gamble, some were duffers.

[g1]

Most people say the fender reverbs sound better than the Boogie ones, is it the extra driver half tube that improves the tone? I can choose whatever tank I like with appropriate impedance etc and I understand how the accutronics codes work for the different types of tanks.

I agree with those people and will even go on to say I haven't ever heard any other reverb circuit that sounds as good as the classic Fender one.
As pdf64 mentioned, the paralleled 12AT7 for current drive is a big part of it. But also the drive transformer, and the 4AB3C1B tank.
So I don't think you should choose any tank you like, a short tank or 3 spring will not sound the same. For the best sounding reverb, copy the classic Fender circuit.

[52 Bill]

But also the drive transformer, and the 4AB3C1B tank.
So I don't think you should choose any tank you like, a short tank or 3 spring will not sound the same. For the best sounding reverb, copy the classic Fender circuit.

I agree 100% with g1. I find that all of the modern tanks that I have heard do not sound the same as the older USA made ones, they all seem much darker and fuller sounding.

I'll add that if you are looking for the sound of a standalone tank with lots of reverb and plenty of splash, then the traditional Blackface will come closer than the Boogie Mark I circuit. But if you just want some added dimension to the sound, then the single tube version will do that fairly well, even with the reduced drive to the tank.

[J M Fahey]

+1 on driving the reverb tank hard.

The only reverb to beat Fender Blackface one, is also Fender made, the standalone one which uses a 4.5W 6K6 pentode to drive the tank even harder than what a parallel 12AT7 can, go figure.


The circuit seems complex but that's because ot's a standalone unit so it needs its built in Guitar preamp and mixer, I bet you can simplify it a lot if you insert it between preamp and power amp or inside the preamp.

Or you can mix the Blackface circuit with this one, replacing the 12AT7 with the 6K6 circuit.

FWIW I HATE the single 12AX7 reverbs, to me the sign of a LAZY and CHEAP designer.
They save 1 triode by reducing drive power to 1 10th of what's needed, and misuse a PREAMP triode like 12AX7 , the worst available for that task.

That in expensive amps, not in $99 Chinese ones.

Oh well.

[Boogie]

Thanks so much everyone, that's exactly what I was interested in knowing. I hadn't heard of the tubes with different types in each half, so now that would be the minimum I'd add, assuming I go ahead with this (which is looking increasingly likely) rather than the single 12AX7 which I was thinkning of as the minimum.

Does anyone have any idea how much spare capacity there would be in the PSU of a 60 watt 1982 SOB (which obviously has no built in reverb already)? It's got two 6L6's and three other tubes including the PI which is a 12AT7 and two 12AX7s. Like enough for one or two more tubes? I've put new PSU caps in it since getting it. I'll email Boogie and see what they have to say. A 6K6 would be SWEET though that would be more extra power, and I'd have to cut a bigger hole in the chassis as they're physically bigger. If it was going to use an actual power/output tube, What about using an EL-84? Which would fit in the same hole that's already there. How different are they? I can look up all the specs.

I was browsing some of the other threads on this subject, which I hadn't seen before. The other thing I was wondering about is that they say that a single gain stage of a 12AX7 isn't enough to bring the tiny spring return signal up to close enough to the same level as the dry signal and either further amplification is required, or attenuation of the dry signal - is this likely to be an issue at all (like would I need to add any extra gain anywhere because of this). I guess worst case would be to use the extra/free half of the return 12AX7. The circuits I've seen I didn't notice anything like that in unless I was reading it wrongly (eg perhaps the dry signal is attenuated to match the reverb return level and then the whole lot gets an extra gain stage that may not be there on my amp?). I should be able to figure that much out by comparing the circuit to a Mark I with reverb schematic.

I've also got a red knob Fender twin which has the standard fender parallel 12AT7 and half a 12AX7. The reverb is definitely better though its not as "out of this world" better as I might have thought, perhaps the red knob version is inferior to the vintage BF though its looks like the same basic design. Perhaps its the actual tank which isnt as good, the original one was a US made one, and I actually swapped it for a different one (i think a MOD based on good reviews) when I had a problem with the reverb (which later turned out to be a failed passive component that I fixed by replacing all 4 or 5 small resistors and caps in the return circuit). The MOD sounded different but Im not sure if better, I might put back the original on and A/B them. From the research I did then it seemed the consensus seemed overwhelmingly the long tanks are better than the short ones.

Someone on another thread was commenting how fender uses linear taper pots for reverb making the usable range only the bottom of the knob's travel, and it should be a linear taper pot - and I've thought that exact same thing with my red knob and considered changing it to a linear one. Has anyone done that or considered it. I also thought that about the master volumes on the red knob since most of the action seems to happen between 0 and 2 and after about 3 there isnt that much difference in volume turning it up higher. (like the taper of the pot is wrongly chosen).

There's an absolutely beautiful looking restored one of those 6K6 standalone Fender ones on eBay Australia atm. I was even wondering how many hours of research and working on my amp am I going to spend putting in a reverb, compared to working at say $50 an hour worth of time to clock up $1200 to just buy it. Its funny how if I had and extra $1200 to spend there would be other things I'd get before a standalone reverb, but I'm thinking (still pretty seriously) about retrofitting it to my own amp which is probably going to take a similar amount of total hours of effort. I guess the reason is that doing this is more interesting than what I get most of my money from (maths and science tutoring) haha.

Is there much of a noise issue with physical placement of the reverb tube and other parts in relation to the rest of the components? Someone on another thread was saying putting it anywhere near the power tube wiring would be way too noisy.

[Boogie]

This looks interesting
(discussing using an EL-84, there looks to be a lot of info out there)

https://www.google.com.au/#q=el84+reverb+driver

I've heard that the fender circuit is really hard on the 12AT7 tubes and they wear out faster than ideal due to using them like a power tube - that would also suggest a larger powered tube for the driver is the way to go

[Boogie]

This is very interesting

http://www.tdpri.com/forum/amp-centr...ver-heard.html

"I just retubed and repaired a Fender Vibro King for a customer. This had to be the absolute best sounding Fender amp I have ever hear. The reverb was simply silky without the hint of spring-y-ness, for lack of a better word. After looking at the schematic it was obvious what Fender did was to build one of their stand-alone reverb units right into this amp. The input jacks not only feed the directly into 1/2 of the first triode but also in parrallel into the input of the reverb circuit, just like you would do if you pluged into one of their stand-alone reverbs. ".............. "Bruce Zinky design !!! I never would have guessed that. I never cared much for his new little amps."

and then later on down the page the OP says

"Now for the reverb tube, this one had a EL84. Certainly a lot beef for a reverb driver tube............ Still this was defintley the best Fender I ever heard."


Then I read a bit more and it seems the EL-84's were driven too hard in the original Vibro Kings and used to die a lot, and that's why they changed to a different tube in the newer ones (I think it was a 6V6). A few people said this was a problem, some said the originals were fine. Then I saw this comment

"In my own VK, I have the original design with the EL-84 reverb driver that I have modified slightly to alleviate the problem many had with the driver tube burning up. Simply reduced the cathode current draw by increasing the cathode resistor value. I am also replacing the EL-84 with a 7189 which is a more rugged version. Sounds great, no worries. " http://www.fenderforum.com/forum.htm...81&offset=5915

This whole thing makes me wonder why an EL-84 would blow up driving a reverb tank when much lower powered tubes can do the job (even 12AX7s) ... Did Fender just get the circuit design wrong with that one. Surely a power tube like an EL84 can be wired to happily provide enough balls to drive those tiny springs, if a pair of them can drive a speaker in a Studio.22 or a PV Classic 30 or a million other amps

This is reassuring re having enough power for at least one more tube (presumably a 12AX7 like the Mark I) "I got an SOB on ebay recently it will arrive on wednesday. Mine is a 1-12 combo. ..... One unique feature of the combo I am getting is it was custom ordered with reverb. I am curious to hear what the verb sounds like." http://forum.grailtone.com/viewtopic.php?f=3&t=33803

[Boogie]

I've been doing some reading and the best option at this stage seem to be the ECL86. It gets better comments than the ECL82. I realise it's not in current production but there are heaps of them on eBay including plenty of NOS ones - do you think this is a bad idea (to use a non current tube)? It may even be possible that have some of them under the house left over from my grandfther's collection of old tube radios since I think I saw them being used for those sometimes.

I saw this circuit (see image below) on another thread, its from a Dumble with an ECL86 reverb, something like this is what I'm thinking of . The reverb is right in the middle of the schematic.

** On this schematic the symbols like V1 and V5 etc are used to indicate the different voltage rails, not particular tubes (unless I'm reading it completely wrong and there is such a thing as four triodes built into the one tube marked as V4 o_O)

The reverb transformer in this circuit is unmarked, will a standard fender one work? The reverb tank in and out impedances are also unmarked but would they be like standard fender ones also?

The voltage marked V5 (powering the triode half of the tube) seems unclear but is it correct that the 12ax7-like half of the tube can be wired just like an actual 12ax7? e.g. using the same B+ rail that powers the other 12ax7's in the amp with the same 100k plate resistor?

The V1 voltage (that powers the pentode side of the reverb tube) is unmarked on the dumble schematic. On my boogie PSU the supply rail before the choke (i.e from the equivalent place on the PSU, thats V1 on the dumble) is 450V, is that about right or too high? I'm thinking it might be too high since on my boogie, from the schematic, the plates of the power tubes say 415V and they are powered in the same fashion as the dumble, from the same place in the PSU going through the OT and on the Dumble the plates of the power tubes say 375V (less than 415V) so presumably that means my version would need to drop the voltage some more where it goes into the ECL86/ reverb transformer, by increasing the values of the 330K, 2.2K and / or 10K resistors, but I'm not exactly sure what those three resistors do exactly (other than provide the correct voltage for the tube) , or what the 22uF cap in there is for

I still have to check out offocially whether my amp's PSU can handle the extra load. From what I've read adding extra tubes taxes the heater supply before anything else though perhaps that only counts for preamp tubes (which have low current)? There's holes predrilled in the chassis for 2 additional preamp size tubes, and the amp can be bought with reverb as an option (which would be a single extra 12ax7), and I'd be highly suprised if the reverb version of my amp has a bigger PT, so it seems a fair guess there's enough power for at least one extra 12ax7 - though the ECL86 is a few watts of power more than that, but not like a heap more......

The 6K6 reverb would be really nice but that would require cutting a new octal size hole for it, and adding 2 tubes (with their heaters etc) rather than one so presumably even more extra power requirements

[Boogie]

[Boogie]

I just noticed what looks like a mistake on the Dumble schematic. On the lower right, the different B+ voltage rails are labelled with what parts of the amp they power. V1 says "plate" and it does go to the plate of the power tubes. V2 says "screen" and "reverb" and it does go to the screen of the power tubes, **BUT** it doesn't go to the reverb, V1 goes to the screen of the reverb pentode (through some resistors)..... something doesnt look right here though my knowledge isn't enough to determine what's wrong....

[66 Kicks]

Whether you take the power from V1 or V2 doesn't matter and it is more typical to take it from V2. What doesn't look right are the resistors and voltages around the ECL86. It looks like it is dropping 15V across a 180 ohm resistor on the cathode. That would be 83 ma which is way in excess of the maximum current for that tube. Then we're only dropping 25V across 10K for the plate. The screen current looks insignificant, so where is all that current going?

This tube has a pretty healthy current appetite. Looks like it idles at 36 ma as compared to 3 ma for a 12AX7 and it wants 660 ma for its heater as compared to 300 for a 12AX7. It can be made to work with a standard Fender transformer and an 8 ohm tank, but it doesn't look optimal.

[Boogie]

Thanks, I'll have a look around for some more circuits. Does it make much difference if the circuit is for using it as a power amplifier (ie to drive a speaker) compared to driving a reverb tank? Before I was mainly trying to find reverb applications but there were several other schematics of them wired as an amplifier driving an OT to a speaker.

[66 Kicks]

Probably all of the standard output transformers could drive a reverb tank, but most of them aren't practical. Once you get over 10W, they just get too big and too expensive and you don't need that kind of power. In the 2W to 10W range, you will want an impedance ratio that suits the tube(s) you are driving it with. For example, there is a champ OT with a 7K primary and with 4 and 8 ohm secondary taps. If you are using 450V with a 7K primary, a 12AX7 won't pass enough current to fully utilize the OT, but a 6V6 will. If you hook an 8 ohm load to the 4 ohm tap, the primary impedance will roughly double. A paralleled 12AT7 will drive the resulting 14K primary. Of course a 6V6 into 7K will put out more power than paralleled 12AT7's into 14K (or a 6V6 into 14K for that matter).

[Boogie]

I found this page with a heap of different output transformers with their specs and what types of tubes they can work with

Valve Audio Amplifier Output Transformers for Single Ended circuits

I emailed Mesa Boogie about excess power supply capacity so hopefully they'll get back to me about that, after which I'll decide what type of tube to use. I'm hoping for something like an ECL 86 or ECL 82 with a few watts of power or at least the 12DW7