Dr. Z Mazerati GT Head -- Push-Pull or Multiple Tube Single-Ended?

[Mike407]

I picked one of these up and it sounds VERY good. Kinda reminds me of a Trainwreck (Rocket or Liverpool, I think) a friend of mine briefly had years ago. LOTS of very defined but smooth distortion when turned up, very dynamic, a great overall vibe.

The problem is, to quote Huey Lewis in Back to the Future, it's "just too darn loud." It has four EL-84s in the power section, so can I safely pull the two middle ones to tame it about 3db? That's not much, but with a sealed 1x10 cab and two clicks on an attenuator, that'd probably be enough for me to have fun with this very cool amp without making my ears bleed.

Attenuator aside, I do understand doing this will affect the speaker impedence the OT is ideal for. Everything (4 ohm output, 8 ohm output, and 16 ohm output) goes UP one setting, correct? So my 16 ohm speaker cab should be plugged into the amp's 8 ohm output if I pull two power tubes, right? (I do know that if I continue to use the attenuator, I might have to make an additional adjustment, but I am only going to attenuate 3.00 - 4.5 db with it.)

Thanks very much for your time and expertise with this,

Mike407

[Jazz P Bass]

At 38 watts rated output I would suspect a push pull configuration for 4 EL84's.

[pdf64]

It has four EL-84s in the power section, so can I safely pull the two middle ones to tame it about 3db?

It depends if they're cathode biased AND share a common bias resistor, eg like an AC30. It would be best to assume it is, as the website states

The GT is cathode biased so output tube changes are a breeze

.
Dr. Z Amplification | Mazerati GT
Anyway, take 2 tubes out of that arrangement and the remaining tubes will lose bias and run super hot, ie it relies on having all 4 tubes conducting to generate sufficient bias voltage across the cathode resistor. It may sound great but not for long!
What you could do is fit a rectifier which will lose some B+, eg 5V4 or 5U4. Pete.

[Mike407]

Ah, the plot thickens . . .

I'm very glad I posted my question here before doing anything. I do NOT want to burn through the power tubes quickly, so thanks for the rectifier-swapping tip.

And yep, it did occur to me, after I posted, that if it were single-ended it would be a) a very unusual design w/ four power tubes, and b) it could produce only 16-20 watts.

For what it's worth, this is a LOUD 38 watts, which makes me wonder how much of a beating the power tubes are taking regardless. I think of four EL-84s as producing an honest 30-35 watts, a la the Peavey Delta Blues, but this amp is more like (or maybe slightly more than) JTM-45 volume, I think.

I do not plan to crank it, as there is plenty of creamy crunch and sustain with the one volume control set anywhere between just 10:00 and 2:00, even with single coils. With my ES-335 copy it begins to break up with the volume at 9:00! At noon it is sustain city, but man, the volume.

I would imagine wide open with very hot humbuckers, this amp would be like a hot-rodded JCM 800, but with more dynamics for sure. It cleans up very well with the volume knob on the guitar. I can even go from crunch to 90 percent clean (with chime!) just by altering my picking. This would be a great no-pedals, plug-and-play amp for British blues and classic rock.

Thanks again to all of you for your input and expertise, as I know next to nothing about electronics.

Mike407

[jmaf]

The Google replied this:





Cathode biased, push-pull. If had just the schematic to look at I'd guess 30 Watts RMS.

[pdf64]

Thanks jmaf; yes, 38 watts seems optimistic - I wonder what the OT p-p impedance is? My old AC30 struggles to manage 25 watts (which is plenty!).
Mike, your amp has got a common 50ohm bias resistor, shared by all output tubes, so it needs all tubes to be fitted / operational. Pete.

[jmaf]

^^^ Yeah. Best option without modding the amp is an attenuator. Or you can lower the voltages, but that would do more to the tone and headroom than to the SPL.

[Mike407]

Maybe if it's too loud, then I'm finally too old!

I have a handful of amps in the 18-35 watt range (Carmen Ghia, Garnet Lil Rebel PA, Kendrick Black Gold 35, SF Deluxe Reverb, SF Vibrolux Reverb), and I swear this seems louder than the others. Perhaps it's the degree of distortion playing a psychoacoustic trick on me. I'm used to being able to get that much sustain and crunch only with either a pedal or master volume amp.

Yeah, either I need to go just with the attenuator a few more clicks, add a light OD pedal, or consider selling or trading it. When I got it, it wasn't the main object of my desire in the trade. It was thrown in as a "balancer."

It was afterwards when I checked it out that I started to like it more than I thought I would-- the tone is infectious.

Thanks again,

Mike

[jmaf]

The loudspeaker has a lot to do with our perception of loudness as well. A few years ago I had to sell my Laney LC15 to pay some urgent bills. It was a loud little monster.... In the shop I used to work at LC15's arrived every so often for repair. I noticed none of them were as loud as mine....I had a look at their circuits and took photos for comparison...I noticed they were newer than mine but the circuit was about the same, I didn't notice major changes... It turns out the later models came with Invader HH speakers and mine was a Celestion 10". The Celestion was brighter, clearer, had less bass and more definition. You couldn't stand in front of it. The Invader was bassier, not quite as loud and overall browner than the Celestions. Soon as I placed an ad for a Celestion-equipped LC15, it flew out the door within days....

Laney has gone back to Celestions since. Laney Amplification

[Gaz]

Mike,

I'm not sure how handy you are inside amps (it seems like you know a fair share), but it wouldn't be a huge mod to add separate the cathode resistors for each pair (100R/10W or so for inner and outer), and bypass caps (you can halve them or keep them the same). There seems like more than enough room in the chassis to mount the additional pair of cap/resistor, especially on the control side on a couple terminal strips.

I played one of these amps before, and it was awesome, but then I tried to build one, and it had way more gain than the one I played. I think my build was correct, and I think the one I played maybe had lower gain tubes or something. Never could figure it out....

And Mike, if you ever get inside that thing could you please take some voltage measurements? I'm especially curious about the plate voltage. Thanks.

[jmaf]

+1 for voltages request

[GainFreak]

Sometimes I'm really amazed how people/companies are able to sell entry DIY level tube amps at almost $1k per face panel knob.

[jmaf]

Sometimes I'm really amazed how people/companies are able to sell entry DIY level tube amps at almost $1k per face panel knob.

It's a well built, hand crafted amplifier and the parts are all good quality. As soon as custom amp shops start competing with China on parts and labor, they're gone.

[GainFreak]

It's a well built, hand crafted amplifier and the parts are all good quality. As soon as custom amp shops start competing with China on parts and labor, they're gone.

There's no doubt about. After all it's the simple rule of supply and demand in action.

[J M Fahey]

+1 on Gaz's suggestion.
The least invasive mod would be to just replace that 50 ohm resistor with a similar 100 ohm one, feeding both center cathodes, and wire both external ones to a switched 100 ohm resistor and matching capacitor.
Warning: it will definitely thump when turned on; maybe not so when turned off , because of the time constants involved.
Contrary to the most popular opinion, I would suggest *not* to change speaker taps "to match impedance".
You are not trying the technical fit of building an exact half power amp (which would *not* sound half as loud) but plain taming a too loud one, so the additional mismatch introduced by simply switching 2 tubes off and not compensating will actually help you.
After all, early Boogies did just that. Nobody complained.