quieter Dumble for recording/small gigs

[norburybrook]

Having built my two lovely amps a 50w and a 100w they're both quite loud when turned up to their sweet spot. This is fine on a gig or in a studio where I can isolate the amps but for smaller gigs or recording with an ensemble I'd get more use from a smaller wattage amp.


So to my question.

If I take my 50w 102 build, can I rebuild the output section so I have 4 6v6's and have a half power cathode lift switch like on my BM to lift 2 valves?

This would give a 22w amp I believe.


My chasse is already cut out for 4 valves so that's not a problem , just need to know if the current voltages/bias would need to altered for 6V6's


Cheers

Marcus

[martin manning]

Sure you could do that. You'll have to re-bias, and you will have more drive voltage than you need for 6V6's, but you have a master volume to address that. You can try the 22W version out without any rewiring by installing 6V6's, resetting the bias, and miss-matching the impedance selector down one click. With the lower current draw the anode voltages will all go up, and that will change the tone some. I would recommend using JJ 6V6's for this.

[renshen1957]

Having built my two lovely amps a 50w and a 100w they're both quite loud when turned up to their sweet spot. This is fine on a gig or in a studio where I can isolate the amps but for smaller gigs or recording with an ensemble I'd get more use from a smaller wattage amp.


So to my question.

If I take my 50w 102 build, can I rebuild the output section so I have 4 6v6's and have a half power cathode lift switch like on my BM to lift 2 valves?

This would give a 22w amp I believe.


My chasse is already cut out for 4 valves so that's not a problem , just need to know if the current voltages/bias would need to altered for 6V6's


Cheers

Marcus

Hi Marcus,

What's your OT primary's impedance, and the voltage on the plates and the PT current? Jim Kelley produced (and Suhr is producing again) a 60 W 4 X6V6A with 4k3 OT and 490V on the plates. However, with 390V on the plates (depending on the PT HT of much lesser current capabilities) you might only get 25 W with 6L6, EL34, or KT88.

Best regards,

Steve

[norburybrook]

The transformer is the Hammond bassman upgrade with Primary Ohms 4200 CT.

Voltages are/were around the 450vDC on the plates looking at my old post notes.


Martin, looking at the JJ 6v6's they seem to be a cut down 6l6 rather than a true 6v6. I can get groove tube 6v6 on a special offer at the moment, any reason y9u suggest JJ's over other 'true' 6V6?



marcus

[Charlie Wilson]

The transformer is the Hammond bassman upgrade with Primary Ohms 4200 CT.

Voltages are/were around the 450vDC on the plates looking at my old post notes.


Martin, looking at the JJ 6v6's they seem to be a cut down 6l6 rather than a true 6v6. I can get groove tube 6v6 on a special offer at the moment, any reason y9u suggest JJ's over other 'true' 6V6?



marcus

They can take punishment like no other current production 6V6 and 450V is punishment.
CW

[martin manning]

CW is right, and since the idle current will be nearly half of what it is with 6L6's installed, the anode voltage will only increase further. Miss-matched by one step you'll be running 8k4 primary impedance, which may not be enough to keep you out of the red zone with "ordinary" 6V6's. The JJ's are nice sounding tubes. I have been running them in my Marshall Studio 15 for several years now.

Remember that epic from last fall where a guy had to complete a 4x 6V6 Dumblesque Brown Note 44 in three days to take on tour? That amp was running ~450V on the plates.

[renshen1957]

CW is right, and since the idle current will be nearly half of what it is with 6L6's installed, the anode voltage will only increase further. Miss-matched by one step you'll be running 8k4 primary impedance, which may not be enough to keep you out of the red zone with "ordinary" 6V6's. The JJ's are nice sounding tubes. I have been running them in my Marshall Studio 15 for several years now.

Hi,

I second Martin's recommendation for the JJ 6V6. Unless you can find a quad of Bendix 5992 tubes, nothing else is as rugged as the JJs.

The voltage on the plates will drop when under a full load, but few current manufactured 6V6 will dissipate 14W, except maybe TAD. TAD 6V6GT-STR have been given a few good reviews but I haven't tried them yet, as to high voltage, supposed to be good up to 450V.

Stick with the JJs 6V6 one of the few tubes I would describe as indestructible.

[norburybrook]

Ok, I'll give this a go with the JJ's, should be interesting experiment.

I might do low plate conversion as well just for fun as it's only a couple of resistors and caps and it seems it's better for single coils which is my main stay.

Anyone give me some ball park resistor values to get the bias voltage in the right range?

I'm on the road right now but if I remember correctly I think I've got 1k2 resistor on the board and 15k on the pot at the moment.


Thanks

Marcus

[martin manning]

You may have the range to bias 6V6's already. I'd just plug them in, and start with the bias at its most-negative voltage. Power up with your DMM plugged into the bias test jacks and see where it lands. In the end you'll want something around 0.7*12/460 = 18mA.

[norburybrook]

You may have the range to bias 6V6's already. I'd just plug them in, and start with the bias at its most-negative voltage. Power up with your DMM plugged into the bias test jacks and see where it lands. In the end you'll want something around 0.7*12/460 = 18mA.

bias test jacks !! nothing so flash on my amp Martin only good old 1r resistors to a ground lugs.


thanks anyway I'll order a pair of 6v6's and check where the current bias is.


Marcus

[martin manning]

Got some mini-grabber leads for your meter? Very handy for hands-off measurements.

Here's a link to the 4x 6V6 BN thread, direct to a post with complete voltages: http://ampgarage.com/forum/viewtopic.ph ... 529#319529

[norburybrook]

thanks...I like the danger element of dangling my right hand into potentially Killer voltage areas left hand in pocket!


I've got some small crocodile clip leads I can attach to my DMM probes. I should actually get a full set of good probes for my meter. It's a very good meter, not a Fluke but seems like it's modelled on it, only came with one set of probes though.


IIRC I've a feeling my 102 is at the limit of it's bias setting now so I think I will need to change resistors to lower the range.

Is there some sort of formula to calculate resistor values for voltage lowering?


Marcus

[martin manning]

Its tricky to calculate, so most people just rely on trial and error. I think changing the 3k3 before the rectifier to 1k8, or increasing the 27k to ground to 47k should lower the bias voltage 6 or 7V.

Using test jacks, or clipping the leads on the 1R current sensing resistor, makes it easy to observe the bias current (voltage) while you adjust the trim pot.