Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 22, 2019, 04:33:10 pm
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anyone have any experience with these amps? I just had one walk into my house.. It needs some work for sure.. the lap steel guitar is in great shape,, the amp not so much not in bad shape but not mint or outstanding..
https://bb.steelguitarforum.com/viewtopic.php?p=2125818&sid=1789b58bf0d46ac78caafb84ef428d66 (https://bb.steelguitarforum.com/viewtopic.php?p=2125818&sid=1789b58bf0d46ac78caafb84ef428d66)
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I got all the mouse droppings and dirt debris cleaned out of the cab.. Here are some pics of the amp parts..
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more pics the preamp uses two tubes a 7F7 and a 1273
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The OT is mounted on the speaker... I don't know yet if it is a field coil speaker..
The Power Amp has a 5U4G rectifier, a 6SL7 PI and two 6L6G power tubes..
Two 10/10uf cap cans and one 15/15uf cap can.. should be able to find some to replace ..
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gut shots
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more
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Those guts remind me of some old Hammond organ amps.
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I found a stamp inside the cabinet that says 1948. And I kind of thought the little cloth pouch on the back panel was cool. Must be for cord storage..
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i love that cab but it looks fragile,handle with care.
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I'm not familiar with Loctal valves and sockets.. can someone tell me how does one go about removing a loctal tube from a loctal socket?
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http://antiqueradios.com/forums/viewtopic.php?f=10&t=199741
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Thanks these two preamp tubes came out easily.. sockets and pins look good. Not much in the little preamp box. One of the pots is very stiff..
OK I read this amp is very similar to the Gibson BR-1 amp. https://el34world.com/charts/Schematics/files/Gibson/Gibson_br1.pdf
Oh and the PT or driver as some call it is a 6SN7 not a 6SL7.
The speaker is a field coil speaker.
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Inside the preamp 2 electrolytics 8 and 10 uf 4 film caps and a handful of resistors,
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In the second and third pic I posted you can see a light green/greyish ceramic resistor with a center tap.. the insulators on the ends just crumbled and fell off when I tried snugging it up to the chassis. I made two insulators out of some leftover turret board material. What is interesting is there is ground on the end of the resistor and the center tap goes to what looks like the center tap on the PT secondary B+. The resistor is tapped at 2.5k ohms. I can't think of any other reason to do this then to lower the B+.
The Gibson amp doesn't have this... or can't see it.
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> there is ground on the end of the resistor and the center tap goes to what looks like the center tap on the PT secondary B+.
This makes a negative voltage. It probably traces to power tube grids. It is nearly a standard self-bias cathode-bias stage except the "cathode resistor" sits "below ground", and carries the current from all the small tubes and possibly the field coil. This "back bias" connection sometimes reduces the filter capacitor cost, in 1940 economics.
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PRR - Thanks for the information, however tracing things out in the ps chassis I do not find what you have described.
My tracing reveals there is about 75 ohm per side of the HV ac taps at the PT to the center tap. The center tap has a green wire that connects to the slider on the 5K ceramic resistor. The end of the ceramic resistor has a white wire that goes to ground. The HV ac taps connect to the rectifier.
The cathodes of the power tubes are connected together and there is a paper 10 uf electrolytic in parallel with that connection. The cathodes are then connected to a 200 ohm ceramic resistor and the other end of that resistor is connected to ground. That is the cathode bias.
The power tube grids are connected to their respective .05 uf aerovox film capacitors and then to the PI plates. Note one power tube grid pin 5 is connected through a 270K resistor across the socket to pin 1 and then through a 18k resistor to 6SN7 grid pin 4. The other power tube grid connects to 6SN7 grid pin 4 through a 270K resistor. I assume this is the signal phase inversion. Pin 1 of both power tube sockets is connected to ground. So essentially power tubes pins 1 and 8 are connected through the cathode resistor.
The white wires you see in the chassis are ground wires and all connect back to a ground lug near the power tubes and they are connected to gnd at the cap cans to the chassis.
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Here are some clearer photos..
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I've got 565 VAC on each PT HV tap to ground unloaded. That's kind of high. I am going to try adjusting the big ceramic resistor on the center tap. But.. I have gotten no reading from one end to the other, so it may have an open spot ..
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Well that was a bad move.. the resistor tiny wire is broken in the middle and now it is worse. The wire was pretty scorched on the tap, I think this resistor has seem some heat, high current.
Question for Sluckey or anyone else. When I was doing my Baldwin the first time the voltages were very high like this Vega.. you said we have ways to bring the HV AC down to a more comfortable level. Would a 200v 50w Zener on the PT center tap do that? Or should I look at dropping the AC input to the PT.. ?
On the variac I input 88vac to this PT and get 400vac on one AC hv tap.
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Ok I'm thinking maybe when this is all hooked up, to the speaker and the preamp all the tubes in, the volts may drop down with the load.
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I would spend some time drawing a schematic of the power supply and power amp.
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Sluckey.. I could do that.. but why.. I've verified it is almost exactly like the Gibson BR-1 ps - one difference is instead of the PT HV center tap going directly to ground it goes through a variable resistance that was set at 2.5K ohms. Other differences are component values. Like 16uf lytics instead of 20uf off of the rectifier lead going to the field coil and coming back from it. I posted the schematic.
Look at the Gibson BR-1. Tell me does that look like what PRR was talking about?
It's also quite possible someone added this big adj. ceramic resistor later in the amps life.. If I could find a picture of the guts of another one I could verify that.
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I'm not real gifted with this stuff, so I find that drawing a schematic helps me understand just how everything works and goes together. It also helps me to have that drawing when I'm doing troubleshooting, even on something I've designed and built myself.
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Respectfully and without being snotty .. if you and PRR don't want to take a minute and look over the Gibson BR-1 schematic, which is pert near the same amp, and maybe answer some of the questions I have presented then I guess there's nothing more for me to do here.. I'm not going waste time on a drawing a schematic of a power supply when I have a schematic of it right in front of me that visually presents it.
I appreciate all the help and wish everyone the best.
I'm good with that. Bye.
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John the Gibson BR-1 is basically the same.. I mentioned this. I appreciate your input.
So, for anyone that might come across one of these or for those interested. I plugged all the tubes back in and plugged in the preamp and the speaker and set my variac at 105 vac input to the amp.
Turned the amp on.. with 70 year old electrolytics and probably tubes. I have 479-0-479 at the PT HV ac. I have 317 vdc coming off the rectifier to the first filter cap. I have 267 vdc to the 6L6G plates.
I played on it a bit, it is rather quiet and had to turn up the volume to 8-9 to get not real loud sound. It sounds good surprisingly. Who knows how efficient this 70 year old speaker is.
My concern would now be the input AC to the 5U4G rectifier.. I took the variac up to 116 vac and the input to the rectifier increased to 496 vac. I think the tube data says 475 max for capacitor input and 550 with a choke input. So I think I know why that adjustable resistor is between the PT center tap and ground. However with the field coil coming right after the rectifier I'm going to go out on a limb and call this amp a choke input amp.
Guess I'll get a new resistor or use a zener..
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Respectfully and without being snotty .. if you and PRR don't want to take a minute and look over the Gibson BR-1 schematic, which is pert near the same amp, and maybe answer some of the questions I have presented then I guess there's nothing more for me to do here.. I'm not going waste time on a drawing a schematic of a power supply when I have a schematic of it right in front of me that visually presents it.
I appreciate all the help and wish everyone the best.
I'm good with that. Bye.
I figured that would be your response. Offer zero assistance then say that.. pretty nice of you.
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Respectfully and without being snotty .. if you and PRR don't want to take a minute and look over the Gibson BR-1 schematic, which is pert near the same amp, and maybe answer some of the questions I have presented then I guess there's nothing more for me to do here.. I'm not going waste time on a drawing a schematic of a power supply when I have a schematic of it right in front of me that visually presents it.
I appreciate all the help and wish everyone the best.
I'm good with that. Bye.
I figured that would be your response. Offer zero assistance then say that.. pretty nice of you.
What? You still here?
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Sure Steve you know I realize you are probably stretched thin helping people. I can get along without expert advice.. I am sure you don't respect me much so I try not to bother you or anyone use.. but I did ask some questions and I thought maybe you might answer.. I should have done a little more digging and work instead of bothering you. sorry.
My motivation for this thread was to put this rare amp out there.. for people that might need to see it and learn something about it. I looked all over the internet and didn't find much so I decided to post this.. I wasn't selfish about it. I ain't making no money on it. When the next guy comes along with a Vega you'll probably jump right on it.. which would be great.
Have a good night.
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Sure Steve you know I realize you are probably stretched thin helping people. I can get along without expert advice.. I am sure you don't respect me much so I try not to bother you or anyone use.. but I did ask some questions and I thought maybe you might answer.. I should have done a little more digging and work instead of bothering you. sorry.
No bother at all. Actually, it's a pleasure. And I did relearn something... I had forgotten about the loctal trick until you asked. So, we both got a bit of benefit.
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mresistor,
I just PM'd you about one that I'm trying to get back into working condition.
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a variable resistance
I can't "see" sh :cussing: t without a schematic, but hum balance? :dontknow:
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Rockerforhire would like to know if that is a good amp tech in the San Diego area. Another tech says the field coil speaker is bad. Probably needs all new electrolytic capacitors to due to age.
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I can't help with advice on a San Diego tech as I'm at the other extreme end of the country. But this article in VG is interesting, and relates to the earlier discussion of the history of Vega:
https://www.vintageguitar.com/27540/vega-model-120/ (https://www.vintageguitar.com/27540/vega-model-120/)
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However with the field coil coming right after the rectifier I'm going to go out on a limb and call this amp a choke input amp.
that would be incorrect, if this is indeed a gibson BR1, as per schema linked in previous post, this is C input filter network.
--pete
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Thanks Pete. I think I did finally conclude it was cap input too. I did draw out the PS end of the amp and found that it was slightly different than the BR-1. My drawing sucks though.. drawing isn't my forte.. and I don't have pics to share of the completed amp as my SD card became corrupted and I lost a lot of pics. Sad and needless to say I will be backing up pics more religiously now.