Marwatt Questions

Wayne_Alexander · Post by **Wayne_Alexander** » Sat 11/27/04 12:12 am

Well, I got it finished. I had a horrendous shriek, independent of the volume controls. It turned out that the ground wire for my "normal" channel input jacks wasn't soldered to the ground bus, and there was a weird grounding problem with the Marwatt channel input jacks as well, but when I removed them and reflowed all the solder joints it was fine.

Issues and questions: I'm running a 12ax7 in V1, a 5751 in V2 and a 12AU7/ECC82 in V3. The "normal" channel is very clean and not very loud, somewhat less loud than a normal 18 Watt, and a lot less loud and dirty (at full volume) than the Marwatt channel. I presume that's because of the 12AU7? Can anything be done to make that channel closer in volume to the Marwatt one?

The Marwatt channel sounds quite good, but the tone controls don't seem to do anything at all. Overall that channel is quite bright in a good way, but it could use some more bass and I wish the tone controls did something. I've used audio 500K pots for bass, mid and volume and a linear 250K pot for treble. Should I use different pots? Should I try different cap values?

I built it with this layout http://www.18watt.com/storage/bm_marwatt_layout.pdf
but added the missing ground wire from the junction of the two 470K resistors on the left, and added the diodes from ground to the plates of the power tubes.

markd · Post by **markd** » Sat 11/27/04 3:28 am

Wayne_Alexander wrote: Issues and questions: I'm running a 12ax7 in V1, a 5751 in V2 and a 12AU7/ECC82 in V3. The "normal" channel is very clean and not very loud, somewhat less loud than a normal 18 Watt, and a lot less loud and dirty (at full volume) than the Marwatt channel. I presume that's because of the 12AU7? Can anything be done to make that channel closer in volume to the Marwatt one?

Wayne,
Have you tried a 12AT7 in V3? Do you have to use a 12AU7 in V3 to tame the gain of the Marwatt channel? You really should have a 12AX7 in V3 for the Normal channel to do it's thing properly. If that causes the MarWatt channel to squeal you could try and lower the gain of the MarWatt preamp a bit.
There are two ways that I would try and accomplish that. You could lower the value of R31 to as low as 100K( I would remove R27 altogether if I did this).
Or you could split the plate resistor(R25) into two plate resistors in series(that add up to the original value of 100K) and take the feed to C16 from the junction of the two. If this isn't clear, look how I lowered the gain in my Vajra TMB channel here:

http://www.18watt.com/Storage/Vajra%2018%20scheme.jpg

This is a trick that Leo Fender used on the Concert 6G12. The smaller the value of the resistor on the power supply side, the lower the amplitude of the output signal, without changing the tone(as long as the two plate resistors, in series, add up to the original value of 100K). You could try two 50K resistors for a start.

Alternatly, you could try a 12DW7 in V2 and a 12AT7, or even a 12AX7, in V3. This will tame the MarWatt channel a bit but with the hotter tube in the PI it shouldn't be too noticable. And it will allow the Normal channel to hit the power tubes harder(like the original 18watt). Just a thought. If you like the 12DW7 in the MarWatt channel you might consider changing the plate and cathode resistors on the 12AU7 section of the 12DW7 tube (pins 1&3 of V2). This is necessary to get the 12AU7 section operating properly, IMO.
hth, markd

Post by **markh** » Sat 11/27/04 12:34 pm

Jeff Swanson (or his partner Bob) will have to speak to this, but the traditional Hiwatt tone stack uses a 250k-B (lin) for treble, 100k-B (lin) for middle and a 500k-A (log) for bass. See, e.g., http://mhuss.com/Hiwatt/Schematics/DR_4InputPre_v1.pdf
The controls in this topology have *way* more effect then the normal Fender/Marshall tone stack.
(Hiwatts are a particular fondness of mine

).

--mark h

SpentMusic · Post by **SpentMusic** » Sat 11/27/04 3:20 pm

Hi Wayne,

You should be able to run 12ax7's in all 3 positions. In the early MarWatts I did put in the 12Xu7 to bring down the gain but it also worked with a 12ax7. The pots in the early version of the tone stack were as close to what I could find that were in my HiWatt DR201 (1977 era) where this tone stack came from (no different then the 504, 103 or 405 for that matter).

We have changed a number of area's in our production amp and this included changing the treble pot to a 500K, whilst the Mid is still 100K and the Bass is still 500K. Bob (my partner that really understands electron flow) has done a number of changes to the PI, Stage I gain and the power section to get the DST MarWatt ready for production and to meet safety requirements. So just just changing to a 500K pot for the treble may or may not produce the effect you are looking for.

I'll tell Bob about this thread and see if he has any other suggestions.

Jeff Swanson
SpentMusic
www.dst-engineering.com
www.swansoncabinets.com

Wayne_Alexander · Post by **Wayne_Alexander** » Sat 11/27/04 7:29 pm

Well, I've now gone to 12ax7's (Mullard) in V1 and V3, with the 5751 still in V2. I've swapped tone pots so there's still a 500K audio for bass, a 100K linear for mid and a 500K linear for treble. I reduced R31 to 100K but left the 200K in R27 after trying several combinations. I also tried Mark's voltage divider trick for the plate resistor, using a 33K on the power supply side and a 68K on the tube/plate side, again after trying several other combinations.

At present, the Marshall channel sounds like you'd expect, a bit dark, and the Marwatt channel is very bright and a lot more gainy (50%?) than the Marshall one. The tone controls have only an extremely subtle effect - mainly, it's slightly throatier and gainier with the treble pot all the way down, very slightly clearer and brighter with it all the way up. The Marwatt channel is a lot gainier than the TMB channel on the "standard" TMB layout that others are using.

I'm going to try this at band practice next week and report back then.

I still wish the tone pots had a noticeable effect on the Marwatt channel - they do far less than even the single tone control on the Marshall channel.

I could probably do with a bit more gain-control tweaking on the Marwatt channel, and I need to figure out how to wake up those tone controls.

SpentMusic · Post by **SpentMusic** » Sun 11/28/04 1:03 pm

Hi Wayne,

Something is wrong on your tone pots. You should have a very noticeable effect on tone from each pot???

Can you send a picture of the circuit board and the connections to the controls. We can trouble shoot from that.

Jeff Swanson
DST Engineering

Wayne_Alexander · Post by **Wayne_Alexander** » Sun 11/28/04 4:54 pm

Jeff, thanks for the help you're offering. In case the photos don't upload well enough, you can see them at this site: http://community.webshots.com/album/223780403bqFXqJ.

I've posted a shot of the whole board and two shots of the pots on the Marwatt channel.

Let me know if you see anything.

Wayne_Alexander · Post by **Wayne_Alexander** » Sun 11/28/04 4:55 pm

Here's the whole board

Wayne_Alexander · Post by **Wayne_Alexander** » Sun 11/28/04 4:56 pm

Here's the detail on the pots on the Marwatt channel

DSTengineer · Post by **DSTengineer** » Mon 11/29/04 7:40 am

Hi Wayne:
The reason your tone stack doesn't work is most likely because the .047U cap that goes to ground with the bass pot isn't gorunded. This will also make the channel sound bright because all of the signal passes through the treble pot/caps. Double check this ground.
As for the squealing and tube choices... well, honestly, all I can say is it's a combination of wire layout problems and grounding. This is always the difficult part of amp building. However, once the tone stack is fixed you may find the other problems are less severe as the tone stack eats up a lot of circuit gain.

Bob Dettorre
The amp guy
DST Engineering.

Wayne_Alexander · Post by **Wayne_Alexander** » Mon 11/29/04 11:14 am

Bob, thanks, that was the problem with the tone controls. It also tamed the gain a lot in the Hiwatt channel, so I had to undo some of my gain-taming mods. I've now got 12ax7's (Mullard) in V1 and V3, with the 5751 still in V2. Pots are 500K audio for bass, a 100K linear for mid and a 500K linear for treble. R31 is back up to the stock 470K. I am using a little of Mark's voltage divider trick for the plate resistor to cut the gain slightly to the Hiwatt channel, now using a 68K on the power supply side and a 33K on the tube/plate side. That makes the Hiwatt channel a little gainier than the Marshall channel when dimed, which is what I wanted.

THe Marshall channel sounds like you'd expect, a bit dark, and the Hiwatt channel is much brighter (I like that), and the tone controls have a lot of effect, in fact the treble control will go to the "thin and shrill" region when dimed.

I'm going to try this at band practice tonight and report back after that, but thanks to Jeff, Mark and Bob for the help. You three, and this forum, are great.

Post by **markh** » Mon 11/29/04 11:03 pm

> thanks to Jeff, Mark and Bob for the help. You three,

Actually I think both Marks provided 2 or 3 cents on this one, so that would be 'you four.'

Looking forward to the in-band review...

--mark h (the mustard) speaking for himself and mark d (the Watkins)

HBamp · Post by **HBamp** » Tue 11/30/04 2:26 am

Hello Waine,

On the picture I see that your 150 Ohm 5 W Cathode resistor is mounted to close to the cap. This thing is getting very hot and you will get problems with that. There have to be a minimum of 1/2 " free space from the other parts.

Henk Bergrath
HBamps
The Netherlands

Wayne_Alexander · Post by **Wayne_Alexander** » Tue 11/30/04 3:42 am

Well, I played the thing at band practice tonight. It worked great. The Hiwatt channel is brighter and cleaner than the Marshall channel at any given volume level, so I matched the volume levels of the two channels and switched back and forth between them, and mixed them sometimes, with my Framptone Amp Switcher (since it's in the same amp probably a simpler A-B-Y switch might work). I don't know how much the Hiwatt channel sounds like a Hiwatt, but it is brighter by a notable amount than the Marshall channel, and the tone controls have a different contour than Marshall ones do on the TMB channel of the "standard" TMB I built. I needed a lot brighter amp than the stock 18W and the stock TMB (they have too much bass for my guitars - mostly PAF Gibsons and an SG with P-90's), this did it. I played tonight through a 1x12 Mojo cabinet with a 30W Weber Alnico Blue Dog in it, that's a good match for the amp. I'll try it later with my 2x12 Mojave cab with a G12H and a Red Fang, the tone won't be much different but it'll have a bit more breadth and maybe a little more bass. So far I'm quite happy with the Marwatt and like it the best of all the 18W variants.

SpentMusic · Post by **SpentMusic** » Wed 12/01/04 12:23 am

Hi Wayne,

Glad you are happy with the design and all of YOUR work to build it!!! LOL

It has a very different sound then the normal 18 watt and we are having a lot of success with the amp. Attached is a clip from Patrick Robinson playing around with him 2 x 10 MarWatt.

Jeff Swanson
DST Engineering
www.dst-engineering.com
www.swansoncabinets.com

jersey_aaron · Post by **jersey_aaron** » Thu 12/02/04 10:23 am

Thats amp sounds REALLY good. Is that the natural gain on the amp? Any chance you could upload a clip of the marwatt through a 12" speaker? How does it sound clean?

SpentMusic · Post by **SpentMusic** » Thu 12/02/04 10:31 am

Hi JA,

That is straight into the amp and nothing added for more gain. The MarWatt has more gain then the 18 watt amp. We end up with a gain of over 4 times the normal Marshall as we have to drive the tonestack and it has a lot of loss. But we still end up with the gain almost doubled.

Patrick only has a 2 x 10 with the Blue Pups in it. We are taking a 1 x 12 down to CT on Saturday to do a demo at a recording studio so will get some at that time. Will try to get some on a 2x12 cabinet as well.

Will post up a number of clips to the DST site this week.

Jeff Swanson
SpentMusic
www.dst-engineering.com
www.swansoncabinets.com

jersey_aaron · Post by **jersey_aaron** » Thu 12/02/04 12:53 pm

Good deal. I'm looking forward to hearing them. Play some Townshend riffs!!!

Post by **zaphod_phil** » Thu 12/02/04 12:57 pm

jersey_aaron wrote:Thats amp sounds REALLY good.

Holy cow- JA hasn't knocked an amp that's not a Richie/JA TMB!

In fact he even seems to like it!!! This has really rocked my universe.

Wayne_Alexander · Post by **Wayne_Alexander** » Thu 12/02/04 5:53 pm

Jeff, in the production version do you do any circuit tricks (like the voltage divider treatment of the 100K plate resistor on V2) to reduce gain to the Hiwatt channel from the preamp stages or do you just keep all that gain in the amp? In my current amp, I'm peeling off some of the gain by splitting that plate resistor 68K on the power supply side and 33K on the plate side, and feeding the .01 cap that goes to the Hiwatt channel volume control from the junction of the two resistors. Can the circuit posted up in the 18W downloads section handle all that gain without blocking distortion or other weirdness? Mine works great right now, but if I could increase the gain a bit without weirdness I probably would.

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