Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ndross475 on March 04, 2021, 11:39:39 am
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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 FireUp, 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...
See Voltage chart, V1 on power tubes is shunted... bias resistor turned all I could get and still a little "cold"? Plates are high-ish V1 V2 pins 1 are high...
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...not that there couldnt be a a boo-boo in there somewhere....
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... along with suggested from Metro... I will say this tranny isn't a metro spec its a Mercury so that could be it.... if I were so lucky...
Any questions, thoughts or suggestions are welcomed...
https://njdimages.smugmug.com/SuperBass/
Thanks,
Nick
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are there 1 ohm 1% resistors to ground on pins 8? right now n/a on your voltage list should be some small (mV range) reading if the tube is passing current. once the tube is passing current you can bias them and the plate voltage should go down as the tube is now "doing work"
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See Voltage chart, V1 on power tubes is shunted...
What does that mean?
bias resistor turned all I could get and still a little "cold"?
There should be a 15K resistor between the bias caps. Changing that to 22K will allow you to bias hotter which will bring all the voltages down a bit.
High voltage seems to be a common issue with MM PTs.
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Thx for quick response.
Pins 1 & 8 tied together w/ 1ohm to ground. See pic.
Will try another resistor, that would be the easiest solution...
Thanks,
Nick
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Popped in a 22k in place of the 15k and no change... I tested it prior to install at 23k....
48mv pin 1 resistor 509 pin 3 bias pot adjusted all the way 'down'
Me feels like I have wired something amiss somewhere and just can't see it....
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Popped in a 22k in place of the 15k and no change... I tested it prior to install at 23k....
Really. OK, put a 33K in.
And/or replace the 47K with a 39K or 33K.
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... bias resistor turned all I could get and still a little "cold"?...
48mv pin 1 resistor 509 pin 3 bias pot adjusted all the way 'down'
Is the 1 ohm cathode resistor shared by all 4 EL34, or is there 1 per EL34?
If 1 per EL34, 24.4W 'anode + screen grid' idle dissipation seems rather hot, not cold.
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Guess I should have looked at his numbers rather than just listening to his words.
Yes, IAW your orig voltage chart, your lowest tube (V4) is running 36.7mA with 512V on the plate. That is 18.8 watts which is about 75%. So all your tubes are running too hot.
Put the 15K back in and adjust the bias to decrease the voltage on pin 1/8 until you are running about 70%. That would be 17.5 watts. That would mean about 34mV on pin 1/8.
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This is where the education takes place..
Yes there is a 1ohm per el34...
And I am seeing what changing the resistors is doing to the bias / plate...
When I have the correct resistors and bias the plates are high and V1 V2 pins 1 seem unusually high.. the others are high but don't seem so far % wise from the builders spec.
MM just sent me a 7 page spec on the PT perhaps that's where I screwed up... I will try to attach...
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Putting the amp back to original docs the tube w/ the highest plate current V6, when adjusting the bias pot counter clockwise lowering the #, the lowest I get is 40ma (plate current ?) still too high... pin 3 513v dc and pin 5 -49v dc...
I could change the bias resistor but that prob won't help the plate... if anything will hurt....
Still no real affect on V1 and V2...
I am convinced that I have goofed somewhere, just gonna print another plan and highlight again... maybe I will see a bad solder joint...
On the upside I plugged in and its very quiet, clean sounds good, didn't get too loud.. only have a 50w cab here...
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I told you how to decrease the plate current. Get that right. Worry about V1 and V2 later. Those are easy fixes.
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I told you how to decrease the plate current. Get that right. Worry about V1 and V2 later. Those are easy fixes.
I did put the 15 back an adjusted the bias as low as possible... 40ma is all I could get.. did I miss another step?
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The 1ohm cathode resistors are sampling cathode current, ie anode + screen grid.
At that HT voltage especially, a bias voltage that brings it down closer to 30mA (ie mV across the 1ohm resistor) would be my preference.
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Putting the amp back to original docs the tube w/ the highest plate current V6, when adjusting the bias pot counter clockwise lowering the #, the lowest I get is 40ma (plate current ?) still too high... pin 3 513v dc and pin 5 -49v dc...
You're trying to decrease the plate current, right? So you need more negative bias, right? If so, changing the 27K resistor to 22K should do it. Increasing the 47K will also do it, but it will decrease your range.
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That did it... hottest tube v5 I can manage down to 34.4 @ 516 so right at 70%... coldest V4 30.4ma @ 516 plate.
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Thanks for your assistance yesterday... Here is the latest... after getting in touch w/ a sr. member at Metros forum he suggested that the voltage specs from George’s list came from his spec transformer which is a lower wind... he thinks MM cloned a vintage tranny which w/ todays wall voltage is giving me the 514 vs 498vdc spec. This isn't fact but a theory. His suggestion was to play it... that some people prefer the higher voltages and some prefer lower... He mentioned experimenting w/ the dropping resistors to affect pre-amp voltage... I switched the 10ks to 15k and that did lower it some.... My thinking now is maybe I “tweak” the amp at 110v on the Variac get it dialed in to taste and just use a Brown Box or the like if I prefer the lower voltage sound... unless you see a gaping hole in that plan... I have been looking over the internet to see if anyone has posted voltages from an original super bass and have yet to come up w/ a schematic w/ voltages... But I have learned a lot... Thanks again for all your help!! Will continue updating with progress.
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Regarding the BrownBox idea, what VAC do you measure at your wall outlet and across the heaters?
As they’re marketing heavily to non EEs, I think it would be very beneficial for MM to explain the inevitable consequence of using ‘vintage spec’ winding ratios for power transformers in regions whose modern mains voltage is not also ‘vintage spec’.
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So just doing some testing... my wall voltage (at noon) was 123.4 vac... with the original voltage testing done w. the Variac set for 120vac. I saw 514vdc on pins 3 of the power tubes.. by dropping the variac to 116.5 the plates were seeing 497vdc which was the spec for the build (not law, but the builders spec). At 116.5 I was still seeing 3.1vac at the heaters.
As for MM and the winding of this tranny, I don't know for a fact that its a vintage clone not taking into account current wall voltages so I would hesitate to take them to task... There is still the distinct possibility that I have made a mistake somewhere....
In the end I guess its about the performance of the amp...regardless of the voltage... if the tubes can handle it... what I am reading is the guys with the old high voltage amps are using NOS Mullards for their hardiness and ability to stand the voltage... Thats a pricey solution I am sure,,, I haven't even looked... Time to get the 4x12 out of storage and pi$$ off the neighbors..
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Just curious as to why no one asked the OP to share what voltages he saw on the PT secondary. Did they match the transformer specs? Seems to be the crucial question in determining if B+ is too high or too low or just right.
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Just curious as to why no one asked the OP to share what voltages he saw on the PT secondary. Did they match the transformer specs?...
Mercury Magnetics are notorious for the paucity of the technical info they provide regarding their products. And the OP wasn’t even their customer, so would seem to have little likelihood of having any leverage to prise any further info out of them.
...Did they match the transformer specs? Seems to be the crucial question in determining if B+ is too high or too low or just right.
Given the voltage survey chart provided in post#1, and with respect to the probable intended PT secondary voltages, how could they not be ‘just right’, eg what could make them wrong?
The issue being that the somewhat high HT (in conjunction with the probable OT characteristics) is likely to put the EL34 anode dissipation under stress. This being the nature of early 70s ‘100W’ Marshalls.
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Interesting take on MM. I have always found them to be quite forthcoming with any tech info I have asked them for. But that is neither here nor there. The hookup specs shared by the OP shows a published secondary voltage of 193-0-193. When I asked the OP to measure the AC Voltage at the PT secondaries he reported getting approximately 190vac on each winding. That pretty much lines up with the spec sheet and suggests that the spec sheet was published using today's wall voltage. Of course I am not there with the OP so we are going to have to take his word for it. The question, for me, is how you can take 190vac present at the PT secondary and derive over 500vdc after a bridge rectifier and filtering. Does the math add up? That is the question in my mind. His build is unique to him and has nothing to do with a voltage chart. Just my opinion.
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...The question, for me, is how you can take 190vac present at the PT secondary and derive over 500vdc after a bridge rectifier and filtering. Does the math add up?...
The CT of the HT winding is a red herring, it carries zero load current, hence the winding should be regarded as 386VAC, which with a cap input FWB rectification will provide 546VDC, unloaded.
Due to transformer regulation and dynamic discharging of the reservoir cap, loading will reduce both VAC & VDC.
Hence the reported EL34 anode voltage lines up with everything being as intended.
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Alrighty then.
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Interesting take on MM. I have always found them to be quite forthcoming with any tech info I have asked them for...
Thanks for the insight, I’m just going off the product info on their website and what has been reported in forums.
I’ve never had any interaction with them, either as a customer, or as a tech with a ‘non-idle curiosity’ justification for spec info.
It would be great if they published by default, like Hammond, expected no load voltage, full load voltage and current, and winding resistance for their products. But given the breadth of their range, I recognise that’s a significant task.
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I think what happens with MM is that they supply primarily to companies that have someone on staff that is spec'ing their transformers to order. In the case of their consumer products they are mostly clones of OEM parts and are used primarily for clone builds. For some reason they don't think that it is necessary for that particular consumer to know much beyond how to hook it up. A contributor to the OP's problem for sure. If one has a good relationship with them they are super easy to work with and will provide anything I have ever asked for. Their consumer facing product is priced out of reach for a lot of wallets anyway. They have gotten a little better lately with at least putting hookup diagrams online but their online store front pretty much sucks. On another note - It looks like Triode Electronics bought Classistone and are going to restart production.