dumb tube questions

[davoux]

I got a Sovtek mig 60 for my Son for Christmas. It was described as having a mod that added a little gain. The seller was supposed to send a diagram showing the mod with the unit, but I don't know if I will ever get it. My son says he wants a little less "crunch" ( I assume less gain/distortion). Could I accomplish that by putting in 1 or 2 12at7 rather than the 12ax7 as they have a little less gain? Or, is the gain produced only in the EL 34s? Would that impact the power output or just the tone?

Second question. I need to replace the Mullard EL34s in a Hiwatt he has. It doesn't have auto biasing so I probably want to get tubes approximately the same as the Mullards. I notice tubes often have reference numbers on them such as 28 up to 40. Would a higher or lower number be more likely to be closer to the Hiwatt bias value? Does the higher number represent a higher current draw or a higher internal impedance? Thanks for answering some dumb questions.

[Amp Kat]

I got a Sovtek mig 60 for my Son for Christmas. It was described as having a mod that added a little gain. The seller was supposed to send a diagram showing the mod with the unit, but I don't know if I will ever get it. My son says he wants a little less "crunch" ( I assume less gain/distortion). Could I accomplish that by putting in 1 or 2 12at7 rather than the 12ax7 as they have a little less gain? Or, is the gain produced only in the EL 34s? Would that impact the power output or just the tone?

Second question. I need to replace the Mullard EL34s in a Hiwatt he has. It doesn't have auto biasing so I probably want to get tubes approximately the same as the Mullards. I notice tubes often have reference numbers on them such as 28 up to 40. Would a higher or lower number be more likely to be closer to the Hiwatt bias value? Does the higher number represent a higher current draw or a higher internal impedance? Thanks for answering some dumb questions.

Not really dumb questions there. The best place to try the 12AT7 is in the PI or the last tube before the power tubes. This will cut back on the gain and keep the tone pretty much the same. The first preamp tube will alter the tone more than the others however the right combination of tubes is kinda a black art and experimental thing until you get what you want. I'm almost sure the mod was changing the 100k platet resistors to 220k hopefully not higher.
Tone changes from low volumes using preamp distortion to when the power tubes start to break up then start getting into power tube distortion. When that happens the tone of the power tubes start to take in and Mullards are one of the best.

I think the numbers are matching lot numbers used to match tubes up with. I think there are also some type of transconductace numbers but the wattage rating is the main thing. If you don't have a bias control it would be a good idea to take it to a tech and let him or her(?) bias it correctly after burning them in for awhile. The main thing to watch is the wattage rating of the tube and some EL-34's draw more current than others and may be a different or higher wattage tube that could cause some red plating however, if replacing the Mullards with the same wattage tube usually 25 watts I would doubt the bias would even need to be adjusted however again that just depends on where it is now and if it has drifted over time since they were biased originally.

[Ray Ivers]

davoux,

If I had your Mig 60 and wanted less overall gain from a simple tube swap, I would replace the first two preamp tubes (V1 and V2) with 5751 tubes, available from Groove Tubes or Sovtek/Antique Electronics. If this results in too much gain reduction, you can mix and match 12AX7's and 5751's to fine-tune the overdrive. The third 12AX7 can remain; as this is a master-volume design, the main thing accomplished by swapping in a 5751 here will be slightly less volume for a given MV pot setting (unless your son runs it wide open - you could try it and see). The 5751 is basically a 12AX7 with slightly less gain and is 100% compatible with 12AX7 circuits, while 12AT7 and 12AU7 tubes generally will not perform at their best there - but, as always, your mileage may vary. Power output should not be affected at all.

As far as the Hiwatt goes - I'm assuming that this is an older unit, with the 'HYLIGHT ELECTRONICS' name plate on the rear panel. If so, most modern replacement EL34's will bias up extremely hot - possibly red-hot - even with the bias control set to minimum current draw, unless the bias circuit has been modified. A set of Groove Tubes with the lowest number possible - try to get #1's if you can, and no higher than #2's - will have the greatest chance of biasing up reasonably well; their new EL34M XF2 Mullard replicas should be the closest to your original Mullards (which are wonderful tubes - are you sure they're dead?). I can't really speak to other numbering systems, although if your chosen vendor rates their tubes for transconductance, you'd want the highest number possible - in a stock Hylight-era Hiwatt, you'll always be able to bias it hotter if you want.

I just re-tubed a '73 Hiwatt 200 by selecting KT88's with the highest transconductance, and ended up happy - but I had a bunch to choose from. I've also used #1 or #2 Groove Tubes in many other Hiwatts and Voxes where the owners were experiencing output-tube overheating but didn't want to modify the amp and thus reduce it's desirability/resale value; it's a little challenging, but it can be done

Ray

[davoux]

Groove Tube?

Aren't the groove tubes with the lowest numbers the early break-up tubes? My son wants it to go to near 8 before it starts to distort. By the way its a 1978 DR103 Hiwatt he's using. I think the Mullards are the original and they seem to be getting a little tired.

He was thinking about going to the Electro Harmonix tubes as the were described by "The Tube Store" as being the closest to the Mullards. Are you suggesting this might be a bad idea as they will bias way to hot?

On the Sovtek, Amp Kat suggested the most likely mod that was made, to increase the gain, was changing the plate resistors to a higher ohmage. Are these the resistors connected to the plate on the 12ax7s? I have a very poor schematic but I don't see any resistors on the EL34 plates.

TNX, John

[Amp Kat]

Aren't the groove tubes with the lowest numbers the early break-up tubes? My son wants it to go to near 8 before it starts to distort. By the way its a 1978 DR103 Hiwatt he's using. I think the Mullards are the original and they seem to be getting a little tired.

He was thinking about going to the Electro Harmonix tubes as the were described by "The Tube Store" as being the closest to the Mullards. Are you suggesting this might be a bad idea as they will bias way to hot?


On the Sovtek, Amp Kat suggested the most likely mod that was made, to increase the gain, was changing the plate resistors to a higher ohmage. Are these the resistors connected to the plate on the 12ax7s? I have a very poor schematic but I don't see any resistors on the EL34 plates.

TNX, John

I'm not sure you can get as close to the Mullards because they are what they are and most of the good ones are gone that are for sale. The EH is a relabled Sovtek and lastibility is the issue with those. Wattage is wattage and to get an EL-34 that breaks up earlier it would have to be a lower wattage version and that means the bias will be hotter than normal wearing out your tubes faster. If you want earlier breakup use 5881's but make sure pins 1 & 8 are tied together.

The resistors I was talking about would indeed be the plate resistors of the 12AX7's plates that are normally 100k before the PI splitter. Don't think they would alter the Cathode combination but then again any things possible. There are no plate resistors on the EL-34's. That connection ties directly to the primary of the Output transformer. Careful there bud, very high voltage present !

[Ray Ivers]

John,

Aren't the groove tubes with the lowest numbers the early break-up tubes? My son wants it to go to near 8 before it starts to distort.

Speaking as a >20-year GT dealer, a set of #1 Groove Tubes will be every bit as loud, clean, and dynamic as a #10 set of the same model; they will just achieve that performance at a lower setting of the master-volume control, and will bias cooler for a given bias control setting. The GT "early-late breakup" boilerplate is mainly aimed at non-MV amp users, IMO.

I don't know if you can get EH tubes specifically matched for high transconductance (I've never asked, but I don't believe New Sensor will do it, and they're the biggest EH dealer I know of). If your already-modified amp has had its bias circuit changed to provide sufficient negative bias voltage, you may be able to bias up practically any current-production tube; my goal was to point you towards a tube set that would have the least chance of turning out to be a mistake, assuming a bone-stock Hiwatt bias circuit. IMO you really have to plug a brand-new set of expensive power tubes into an old Hiwatt and watch them turn cherry red to fully appreciate the situation.

On the Sovtek, Amp Kat suggested the most likely mod that was made, to increase the gain, was changing the plate resistors to a higher ohmage. Are these the resistors connected to the plate on the 12ax7s?

Yes. There are at least 7 places in the Mig 60 preamp where gain/overdrive can be increased, so I really don't know what to tell you for sure.

Ray

[Bruce / Mission Amps]

Do you have a schematic for a MIG60?
If so, here's what I would do as an experiment:
Use a 12AY7 or 12AV7 for the first preamp tube, keep the others as is.
Drop a 10uF/25v-50v cap across R3 and for a little more gas, a 1uF/25uF-50v cap across R9.
Now let him try both input jacks and see which he likes.

[davoux]

Bruce

I do have a very poor, barely legible schematic for the Mig 60. If I understand correctly, you are suggesting I replace c22 which is a 47uf bypass cap around r3 with a 10uf. And, then add a 1uf bypass cap around r9 on the other side of the 1st preamp tube which currently has no bypass.

I am just so humbled that so much talent would be willing to help a neophyte with what to you are simple problems. God bless you all!

[Enzo]

Schematic Heaven has the Mig60 drawing. WHile it might be a little rough, it is not "barely legible." Try it.

[davoux]

schematic heaven

To me Schematic Heaven is more like hell. I have never found a schematic there. I always get caught in the "do loop" going from the angel over flames page back to the angel over flames page or out to some page that has no relevance other than it will bring you back to the angel over flames page. I'm sure I'm doing something wrong but on at least 5 times I've tried for at least a half hour each, I've never figured out how to get any info.

[Enzo]

I go there all the time, hell i have even sent him schematics.

At the opening page you see the "album cover" with the angel. Immediately below that are two PLAY messages - low speed or high speed. You have to click one of the PLAY mesages. I always click low speed for my dial up. All that does is select the interface, though someone reported some differences in the resulting menu.

After clicking on PLAY low speed, I get the same apparent album cover, but under it now are flags. They represent other languages. And below them are the menus. Scroll down past "getting started" and "most wanted" and you find the brand list. In the case of the Mig, I look in "Post 70s amps" and there it wil be in the SOvtek grouping. CLick on Mig60 and it should download itself and display it in Adobe reader.

[Amp Kat]

here it is http://www.schematicheaven.com/newamps/sovtek_mig60.pdf

Hope it works it gave me an error report.

[Enzo]

Your link opens for me.

[stokes]

Link worked for me,too.