4x6L6GA in D-Build?

[David Root]

Lots of folks have noted how the 100W Dumbles have a bigger tone than 50W builds, regardless of volume, and some have noted that it is mainly because the 100 Watters use 4 tubes, and it is 4 tubes that gives that "extra girth" which is a good way to describe it. I've built 2 and 4x6V6 amps and the 4 does sound better than two to me.

However, 100W is way too much firepower for many folks (not including me), so it occured to me that it might be interesting to do something around 60W or so but still using 4x6L6. I guess you could do it with 6L6WGBs or 5881s, but I have two really nice old (I mean OLD) NOS 6L6GAs, see pic below. So today I ordered another pair from the same source I got this pair, presumably of the same vintage, I hope so because the price went from $70/pair two years ago to $100 today.

I have heard it said these older 6L6s sound very, very nice. I know you can't put much more than 400V to them, but I'm wondering how the screens will hold up with 400V on them too. There are a couple old tweed Fenders that did that so I think as long as I don't go close on the screen Pd I will be OK.

Anyone used these tubes in a quartet Dumble style? I don't want to reinvent the wheel here. Curious also as to what design would best suit them, maybe a half power SSS?

[guitardude57]

I like all the 6L6 types. You could build whatever circuit you want.

I would just use a smaller voltage PT to put the plates and screens where they really should be.

Then adjust the whole dropping string to get the preamp voltage where they need to be. Duncan's PS calculator is handy here.

I ran high volts on a Sov's pair that were fine until I cranked up the amp. Didn't last very long.

[David Root]

Thanx gd57. I looked thru my iron stock and picked out a Magnetic Components 4 ohm Twin OT and a Mercury Magnetics low voltage 59 Bassman "Fat Stack" PT which is 310-0-310 on the HT winding. This is about right for 400 - 410 VDC B+.

As it happens these items fit nicely the AB165 Bassman chassis left over from my 50W Princeton ODS build, PT just slots right in. And I have a ready head cabinet too!

So I can do something a bit different and recycle the AB165 chassis and head cab.

The only thing I am wondering if the Mercury Magnetics PT has enough DC mA, no rating is shown on the data sheet. Update: No I don't think it does, will use a standard Twin PT instead.

Also, the datasheets for 6L6 show primary impedance of OT either 3800 or 6600 for a pair. I can't see why ~2K wouldn't be OK, these are 6L6s, just not 6L6GC.

[llemtt]

Also, the datasheets for 6L6 show primary impedance of OT either 3800 or 6600 for a pair. I can't see why ~2K wouldn't be OK, these are 6L6s, just not 6L6GC.

at least 10% of your amp tone depends on primary impedance choice...

cheers
teo

[David Root]

I am aware it makes a difference and in power too, but I am having a tough time figuring out how Fender came up with 4200 ohm primary impedance for a pair/2100 for a quad.

[guitardude57]

The data sheets show an optimum primary impedance for OT.
But depending on the tone specs you are seeking, doesn't mean it is perfect on paper....right?

This can be made closer to optimum by speaker mismatch 8 ohm into 4, 4 ohm into 8 etc. But even a mismatch can sound superb. I.E. the Express running EL34's at 5.2K or 6.6K and Marshall at 2.4K (I believe).

On my test circuit amp that runs on a choice of 7189 or 6L6/5881 etc., (each pair cath biased independently) run off a 8K tranny....it was what I had. Either set has it's own flavor and sound great.

At full dime volume I do have a small amount of blocking on 6L6's. So if set to 90% have zero issues, and it is still (way too) loud enough to blast through the Twin and Marshall the other fellers were playing at a jam. Even with Cathode Bias.

When my new Express is complete, the test chassis will become a different amp......again. Swap out the preamp components, leaving the power amp as is for ease.

I'm sure some others will chime in on deeper issues with tube/tranny match/mis-match

[David Root]

I went back and did over my penciled extrapolation from G2=300V to G2=400V on the 6L6G curve, and now I see the 4200/2100, or pretty close, at the knee of the curve. My Twin OT is about 1900, which moves the operating point back a bit above the knee, which is safer than the other way.

So I'm a happy camper since I like 6L6 tone in that Fender Zaa zone.

[David Root]

Cranked it up today, the cleans are very very good and so is the OD channel. Very articulate. Having trouble with the FET input (no output) so I don't know what that sounds like yet.

It does not have much bass. Don't know why, but I ran it into a 2x15 JBL cab 4 ohm to 4 ohm and it is a different animal than my #124 100W. The bass pot does not seem to do much if anything.

Haven't tried it yet on the two Altecs I built it for.

[guitardude57]

Mine is a 50 watt high plate, and on OD2 Cathode put a 1uF instead of the 4.7's I have on the other caths.

Wasn't sure how strong the bass would be so went that route.

I like what I got out of the deal. The skyline EQ seems to be pretty subtle in itself.

I didn't really know what to expect tonewise from this ....as it is my first Dumble. As things break in.......I like it.

The cabinet I use with the amp is an old Marshall 1966A with 2 EVM 12L's and 4 - 2 inch holes in the back to breath a little........no low end probs here.

What are the cathode values you used on yours? Maybe go a little bigger on the caps........or maybe reflow what you have first and see if they aren't grounding enough.
Mike

[llemtt]

having a tough time figuring out how Fender came up with 4200 ohm primary impedance for a pair/2100 for a quad.

probably RCA handbooks

optimizing for max power (and less distortion)