how to tame a loud amp?

[Tonegeek]

In terms of volume (wattage): 2 6v6's are not quite enough for my needs, and 2 6l6's are too much. I like to be able to get a good clean sound, but then also get good overdrive without being too loud.

With that in mind, I am considering a 2- 6l6 Dumble build with mods to get a little earlier limiting/breakup from the power section:

1) Using a tube recto to get some sag when pushing the amp
and/or
2) undersizing the OT tranny with a 40 watt tweed one.
and/or
4) use EL-34's which break up a bit earlier than 6L6's
or
3) going Class A on the output tubes (cathode bias)
5) use some kind of variable control (like London)

Anybody see any issues with the OT option (asside from the obvious overheating issues)? I am thinking doing option 1 and 2 might do the trick without causing smoke because the recto tube would limit the current at the output. Seems the Class A option might limit the wattage too much, but I am not sure. I do like the sound of class A amps. The variable wattage idea is interesting but don't know what that does to the tone and also it looks a bit complicated to implement. Regardless of options, I don't want to stray too far from the Dumble sound.

Thanks,
whit

[Icetech]

I was under the impression that going cathode bias doesn't automatically make a amp class a?

And btw... i have el34's in mine right now.. and its not alot quieter than 6l6's... i like the sound better though:)

[Bob-I]

A few points to make here

1) Dumbles derive their overdrive from the preamp and therefore tend to sound really nice at low volumes. Yes they're better cranked, but I like my 6V6 amp just fine at a reasonable volume.

B) Changing the output to class A will change the charactor of the amp. It simply won't be a Dumbe anymore

4) Fuchs has a pentode triode switch on this transplants that really helps lower the volume.

[jaysg]

In terms of volume (wattage): 2 6v6's are not quite enough for my needs, and 2 6l6's are too much.

If you mean that a Deluxe Reverb(18 to 22W) isn't quite enough, but a Vibrolux(35W) is too much, then your best bet would be to limit the power by limiting the PT secondary for a ~400V B+ with 6L6's. You'll probably get 25 to 30W. Also, you can use a PI that breaks up earlier too, like on a 5e8-a.

Moose's Trainwreck PT might be a good choice.

[Bob-I]

just re-read my post and realized that I didn't help at all

What I was suggesting was a pentode triode switch on a 6L6 amp. Don't skimp on the OT, Dumbles derive their OD sound from other sources.

[Darkbluemurder]

Do you have a global master volume on your amp, i.e. one that controls the volume of both channels? Without this, i.e. with only the OD level control a 2 6L6 amp will be damn loud - I know because my Concert still needs one (I expect to receive the ordered parts tomorrow.

Otherwise I fully agree to Bob-I that those amps sound quite good even at bedroom levels.

[Luthierwnc]

I wouldn't worry about a 6L6 amp being too much. The preamp on these things will give you a very wide range of tones. I would worry about using undersized iron, retro-components or inadequate power filtration in an attempt to warm the amp up. A big part of the sound is a relatively clean power amp that doesn't paint a broad brush over the nuances going into the phase inverter.

I've built two of these amps. The first was a fairly stock HRM head using 7581 outputs with 480 VDC on the plates. It is very much in the "D" amp universe tonally. The second amp was an attempt to do what you are thinking of with a pair of 7591's at 418 VDC. It is probably good for 28 watts and is really loud anyway. I used a JTM45 OT because of the impedance match and to not saturate the core on the low end. There is also a fixed/cathode bias switch for even more warmth. The amp was a very successful in terms of useable tones but it is not a "pure" Dumble sound in the same sense as the head. It is also really heavy so I didn't save myself any trouble schlepping the thing around.

Long story short(ened), be sure of how much of the true Dumble sound you are prepared to lose by going off the reservation. These things are a system and the goal is not a happy accident of pre and power amp distortion. Use high-power iron and recommended components, then make adjustments for different outputs if you still need to drop the headroom.

Good luck, Skip

[Tonegeek]

Lots of good points made here. Luthierwnc point about "going off the reservation" hits home. I do like the Dumble sound and there is a point to be made about having a hefty power section, which is what I like (and sometimes dislike) about my Twin. I also like to tinker and sometimes think I can have one amp or one guitar "do it all", if I just make the "right mods". Realistically, I think I just need to plan my build so I can modify it if I want to, but get a good baseline using guidelines you guys have established with all your hard work. I am off on Holiday which will give me time to digest the info I have gleaned from reading these excellent posts. Then it is time to work on the schematic and layout...Happy Thanksgiving! Whit

[stratcat62]

I'd look into power scaling. Power scaling let you turn the amp down with no tone loss, other than speaker cone breakup loss, but you'll lose that anyway by turning the amp down. It's wired into the amp and works kind of like an attenuator, but directly with the power section instead of after it. By turning the amp's power down you actually end up saving the power tubes because it's draws away from the plate voltage to lower the power. Here are a few websites, one is the company that makes the power scaling circuit and the other is a test of the power scaling with samples at different levels. Hope this helped.

http://www.londonpower.com/

http://bruceclement.com/music/psk.htm

[d95err]

I was under the impression that going cathode bias doesn't automatically make a amp class a?

Yep. "Class A" is usually the marketing hype buzzword for "cathode bias". I guess "cathode bias" is too technical for marketing... There are virtually no commercial Class A amps in existence, except for single ended amps (e.g. Fender Champ) and some obscure boutique amps.

Amps like the Vox AC30 are fairly close to being Class A because of relatively low B+ voltages, but are still Class AB. To get true class A, you need very low B+ voltage, otherwise the tubes will burn. For 6V6s or EL84s, you need a maximum of about 250V from what I have read (see www.aikenamps.com).

[Luthierwnc]

Regardless of which way you go, I recommend you keep one of your power supply options on the higher-end of the voltage range for the tubes (ie 450+ for 6L6, EL34, etc). An essential part of the signature sound is where the power amp is not distorting much -- especially on the clean channel. Those silver Bassmans in the originals were from the years when Fender was getting 50% more power than their tweed-era amps from the same tubes.

Remember, you have five volume controls in the pre-amp: the first volume, the master, the OD trimmer, drive and level. An HRM signal is only about 15k-20k from ground at one point. On a scope, you'll see a very sculpted sine wave by the time it hits the phase inverter. More than any amp I've studied or built, this one is about the preamp. My uneducated guess is that is why you see so many of them using 6L6 tubes -- they cover a lot of ground without playing favorites.

I made a 6K6 version of Glaswerks Reverb-o-loop at the same time I made the HRM head. My favorite tone out of all of them is the clean channel (mine is hardwired for the Rock setting) with the Dumbleator set at just over sonic parity (3-4, 3-4) and a little verb for seasoning. On my rig, it seems to give a lot of separation between the notes. Kick in the OD and it doesn't add so much more that you sacrifice clarity for gain. I don't have a treble cap on the clean master volume which may reduce the overall tone on clean. My loop has bright switches but they don't seem necessary. If you decide to add one of these, you might have a look at the comments Dogears made about reversing the phase. By pulling the signal off the cathode of the first triode and the plate of the second, you are only getting half a twist. I haven't had a problem but I have an unsophisticated set-up. YMMV.

StratCat62 -- thanks for the power scaling information. I'd heard of it for years but wasn't tempted enough to buy the book. Mojave has a proprietary attenuator but I don't know much about that either.

Have fun, Skip

[tictac]

If 6V6's (12w) are not enough power and 6L6's (30w) are too much try some 7591's (19w). They are more power than a 6V6 and less than a 6L6 and they sound great. Different pinout than the 6V6/ 6L6 family of tubes. JJ's makes them so you don't need expensive NOS 7591"s

TT

[d95err]

If 6V6's (12w) are not enough power

I though you would get something like 20-25W with 2x6V6. At least, the Fender Deluxe Reverb is rated at 22W with 2x6V6 and about 420V on the plates. The Tweed Deluxe is rated for 12W, but that's with relativey low voltages and cathode bias.