69 Super Bass Build and Testing; Are voltages right?

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ndross475
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69 Super Bass Build and Testing; Are voltages right?

Post by ndross475 » Thu Mar 04, 2021 8:09 am

The back story: A local amp builder had a set of Mercury Trannys for a Super Bass build, which was on my bucket list and he wanted to get rid of them so I ended up with them. New / never installed. PT MSUPB-100P ; OT 0100JM-S; Choke M-CH. ( I will post docs to the album linked below.). So it was like getting a deal on a boat motor, now I have to build the boat.

First mistake.. I followed the Metro Instructions and sourced most of the small parts from Valvestorm but sourced my chassis and plates from Mojo ( I live close to them and could pick it up). The MOJO chassis layout is slightly different from the Metro Docs hence the fuses being reversed in the build.

The build went pretty smoothly once I got all the hardware installed. The Mojo Chassis wasn't drilled for the Mercury trannys so I had to do a little modding but once all that was done the small parts went together smoothly... I followed the Metro instructions and values to the letter except the resistors on filter caps 2 / 3 are 100k vs 56k... ( I didn't have the 56ks for some reason). I changed the grounding scheme to what's been suggested on this board. But all else should follow spec.

First Fire, heater values came in on point, pre-amp voltages are a bit high, went ahead with power tubes and got it biased in... Voltages still seem to be not in line with the suggested voltages...

I have gone back through the board and double checked all my resistor values to make sure they were correct, I did check all values w/ a meter prior to install so they "should" be good...

Here is a link to where the Hi-Rez images and supporting docs can be found along with current voltage charts 1 no power tubes 1 with...

Any questions, thoughts or suggestions are welcomed...

https://njdimages.smugmug.com/SuperBass/

Thanks,
Nick

danman
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Re: 69 Super Bass Build and Testing; Are voltages right?

Post by danman » Thu Mar 04, 2021 7:16 pm

Nice looking build! What voltages are you concerned with exactly or do you feel the entire amp is running to high? Is the amp performing properly and sounding like a Superbass should?

ndross475
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Re: 69 Super Bass Build and Testing; Are voltages right?

Post by ndross475 » Thu Mar 04, 2021 8:07 pm

Right now the pin3 plates are running at 512-ish... and with the bias pot turned all the way down my plate current across the 1ohm is 40ma....avg... a little hot according to the bias calculator...

pin1 v1&2 seems higher than should be...

As for sounding like a super bass,, I have never had the pleasure.... so not an authority..

Cause the bias is hot and I only have a 50w cab here I haven't cranked it but its the quietest amp I have built and the cleans sound great to me..

When I get it dialed in I have a friend who has / had a Metro 68 Super, was going to get his opinion to see how close to the Master's this one comes...

n

ndross475
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Re: 69 Super Bass Build and Testing; Are voltages right?

Post by ndross475 » Thu Mar 04, 2021 8:09 pm

I posted the lastet voltage chart in the gallery a short while ago...

n

danman
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Re: 69 Super Bass Build and Testing; Are voltages right?

Post by danman » Thu Mar 04, 2021 8:44 pm

Your voltages throughout the whole amp are a touch high but not unheard of for a 100 watt circuit. The voltage chart from Metroamp shows slightly lower voltages but this is only accomplished when using the PT that George specs for his kits. Mercury is known for cloning old PT's exactly which is fine, but with today's higher wall voltages you will end up with a slightly higher B+ throughout the entire amp. Some folks prefer the punch of the higher voltage amps and others prefer the slightly softer feel of the lower voltage circuits. I would test the amp out and see how it sounds to you before making any changes. The higher voltages may eat through power tubes a bit quicker though and many players will opt for NOS Mullards in these high B+ amps because they can take the extra abuse. If you do decide to lower the voltages in the preamp, changing out either the 10k or 8.2k dropping resistor on the board will lower the preamp voltages. Try a 15k in one of those spots and see where your preamp voltages end up. 18k or 20k would also be acceptable if the 15k does not lower them enough. This is one area that you can play with to fine tune the feel and response of the amp.

ndross475
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Re: 69 Super Bass Build and Testing; Are voltages right?

Post by ndross475 » Thu Mar 04, 2021 8:53 pm

Thx I will give that a try.. I changed out the 27k resistor and was able to get my bias down to the 70% mark.. so I feel a little more comfortable messing with it.. Gonna get the 4x12 out of storage tomorrow and see how she does cranked up...
nick

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Re: 69 Super Bass Build and Testing; Are voltages right?

Post by danman » Fri Mar 05, 2021 9:30 pm

I read through your thread over on the Hoffman forum earlier. Everyone's advice was spot on and it looks like you have got your bias into a good range. From the looks of your voltage chart I would say that you have everything wired up properly and the amp should be good to go. It's unfortunate that many manufacturers are providing PT's that deliver such a high B+ voltage but it's not unusual to see some old Marshalls running around the 500v range. Another thing that affects the final B+ voltage is the current rating of the PT. Many old PT's were barely adequate on their current ratings so the voltage would fall a bit once the power tubes begin drawing current. Many of the newer PT's are wound to deliver a slightly higher current rating than their older counterparts. This helps the PT to run a bit cooler but unfortunately the voltages do not drop under load as much as they should. Biasing the tubes a bit hotter will bring the current down a bit but this does cause a bit more wear on the tubes.

I mentioned earlier that a set of old Mullards would handle the higher voltages but you should be fine running a set of new production tubes at a much lower cost. IIRC, many builders are recommending the current production, Chinese made EL34's for their tone and durability.

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