Midland Tube Amp help
- Thread starter Amcrebelfan
- Start date
linuxslate
Well-Known Member
Back in the days of the CB craze, Midland had a fairly good reputation. Yes, there was some cross-branding. I had a couple that where branded "JCPenny", but obviously the same radio.
They are still around, and according to this, they still actually develop stuff:
https://en.m.wikipedia.org/wiki/Midland_Radio
Point is that it could be a fairly well-made amp for what it is. Probably worth doing what rudedogg suggests, as long as the transformers are good.
Amcrebelfan
Super Member
Amcrebelfan
Super Member
It's made in Japan if that helps.
Amcrebelfan
Super Member
Okay the tubes are Hitachi all 3. The 6X4 tests 45 and the two 6BM8's both test 75. No shorts on all 3.
It's has
3x 0.0001uF 600mv
1x10mfd 350v
4x0.005uF 600mv
2x10mfd 25v
1x30mfd 25vdc for caps
It has what I assume are resistors?
RDIL K-420 X2
RD2L K-2 K X1
K100K X4
K250K X2
K15K X1
K50K X4
K-2 M X2
BIG green guy in pic only says 750
It's has
3x 0.0001uF 600mv
1x10mfd 350v
4x0.005uF 600mv
2x10mfd 25v
1x30mfd 25vdc for caps
It has what I assume are resistors?
RDIL K-420 X2
RD2L K-2 K X1
K100K X4
K250K X2
K15K X1
K50K X4
K-2 M X2
BIG green guy in pic only says 750
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dogbone things are resistors, and probably fine. I'd probably leave them be. Most of the time I've had Japanese resistors fine, and still on value. Pretty sure they are an early carbon film type.
I doubt those caps are rated for 600mv, probably 600V, not sure what the m is about. Sounds about right though, 2 filter caps, a handful of coupling caps, a couple cathode caps. If any are those green disc ceramic, those should be fine. Its the grey ones and the filter cans that are a problem.
I doubt those caps are rated for 600mv, probably 600V, not sure what the m is about. Sounds about right though, 2 filter caps, a handful of coupling caps, a couple cathode caps. If any are those green disc ceramic, those should be fine. Its the grey ones and the filter cans that are a problem.
Amcrebelfan
Super Member
Oops I see I missed the transformer caps. I need to see values of those, any recommendations on caps to use?
Amcrebelfan
Super Member
I will check that out I didn't notice any burnt but i might have over looked it.
Gadget73 where do you buy caps from?
Thanks everyone for all the help.
Last edited:
Amcrebelfan
Super Member
I am not finding the caps with 600 MV, should I try different values?
Thank you
Thank you
century tek
Addicted Member
They are 600 VOLT not MiliVolt. You can always go higher in voltage ratings but never lower.
century tek
Addicted Member
I'm not familiar with Mouser or Digikey but I buy my capacitors at Tubes and More which is also known as AES or Antique Electronic Supply.
linuxslate
Well-Known Member
Mouser and DigiKey are large electronics parts and test equipment retailers. They've been around for decades. The other that I remember from my Tech days is Newark, which I now find out is "Newark element14", soon to be just element14.
I think I still have some phonebook size catalogs from Newark from the 90's or early 2000's, and Mouser and DigiKey spam me constantly.
Today, I usually need one's and two's of stuff, and I tend to buy on eBay.
We also have a cool place nearby called MRAM (AKA "Astro Too"). Lot's of random surplus junk, including old stuff. I found (2) 450V Electrolytics for my HK a224 by sitting on the filthy floor for an hour, looking trough a small mountain of loose capacitors. My amp would look better inside if I was more patient, and I had ordered some quality, new can caps via eBay, but with a local place like that, my amp was working the same day, and still is.
Not something someone would do for a custom build or professional restoration, but great for "seeing if it works".
I think I still have some phonebook size catalogs from Newark from the 90's or early 2000's, and Mouser and DigiKey spam me constantly.
Today, I usually need one's and two's of stuff, and I tend to buy on eBay.
We also have a cool place nearby called MRAM (AKA "Astro Too"). Lot's of random surplus junk, including old stuff. I found (2) 450V Electrolytics for my HK a224 by sitting on the filthy floor for an hour, looking trough a small mountain of loose capacitors. My amp would look better inside if I was more patient, and I had ordered some quality, new can caps via eBay, but with a local place like that, my amp was working the same day, and still is.
Not something someone would do for a custom build or professional restoration, but great for "seeing if it works".
Ahhh, yes, andother of the gold-tone amplifiers, all birthed in the same Japanese factory during the Eisenhower administration.
Your unit is identical to the Calrad SA-2B, which is itself identical to units from Olson, Armaco, and a host of other mail-order companies.
While I do not have any documentation on this particular model, as I do for other Calrad units, having studied many of the similar amplifiers (mostly push-pull 6BQ5 and 6BM8) created in that nameless factory and then sold by these catalog companies I can tell you that the circuits are not only nothing fancy, but all have been extensively Muntzed. As a result, the circuits require some simple modifications for stability, reliability, and performance. I therefore suggest you use a decent worked out 6BM8 design as a basis for a rebuild.
The design, in summary, uses the triode portion as a gain stage and the pentode portion as a low-power SEP stage. You should be aware that many of these designs push the power capability of the pentode portion to or beyond its limits, dramatically shortening the lifespan of both the tube and the output transformer.
The original specifications for many of these amplifiers are just delusional if one knows the limits of the tube. I just picked up a Japanese push-pull 6BQ5 amplifier of the same era which used fanciful ratings; 20 WPC (ha!) for a pair of tubes rated for about 16 WPC maximum. The unit shows obvious damage, as the chassis has scorch marks around the tubes, one output transformer is burnt up with scorch marks on the paper, and the other output transformer has obvious heat damage. Tubes and output transformers therefore require replacement, and I will modify it to limit the gain to a more reasonable 16 WPC max.
The Japanese single-ended designs of that time are even more prone to being overdriven because of the low efficiency.
These units do make excellent headphone amplifiers, where power requirements are low, so your plan is perfect. As for driving speakers, well, you'll likely be happier with something of greater power.
Because of the cost-cutting aspects of these designs I suggest you do a few things for stability and lifespan. Off the top of my head:
Your unit is identical to the Calrad SA-2B, which is itself identical to units from Olson, Armaco, and a host of other mail-order companies.
While I do not have any documentation on this particular model, as I do for other Calrad units, having studied many of the similar amplifiers (mostly push-pull 6BQ5 and 6BM8) created in that nameless factory and then sold by these catalog companies I can tell you that the circuits are not only nothing fancy, but all have been extensively Muntzed. As a result, the circuits require some simple modifications for stability, reliability, and performance. I therefore suggest you use a decent worked out 6BM8 design as a basis for a rebuild.
The design, in summary, uses the triode portion as a gain stage and the pentode portion as a low-power SEP stage. You should be aware that many of these designs push the power capability of the pentode portion to or beyond its limits, dramatically shortening the lifespan of both the tube and the output transformer.
The original specifications for many of these amplifiers are just delusional if one knows the limits of the tube. I just picked up a Japanese push-pull 6BQ5 amplifier of the same era which used fanciful ratings; 20 WPC (ha!) for a pair of tubes rated for about 16 WPC maximum. The unit shows obvious damage, as the chassis has scorch marks around the tubes, one output transformer is burnt up with scorch marks on the paper, and the other output transformer has obvious heat damage. Tubes and output transformers therefore require replacement, and I will modify it to limit the gain to a more reasonable 16 WPC max.
The Japanese single-ended designs of that time are even more prone to being overdriven because of the low efficiency.
These units do make excellent headphone amplifiers, where power requirements are low, so your plan is perfect. As for driving speakers, well, you'll likely be happier with something of greater power.
Because of the cost-cutting aspects of these designs I suggest you do a few things for stability and lifespan. Off the top of my head:
All of the Japanese electrolytic capacitors used for AC interstage coupling are bad and will consequently place a DC voltage (from the triode gain-stage stage's plate) on the grid of the 6BM8's pentode stage, blowing up the tube. These capacitors must be replaced if you want a stable amplifier. None can be saved. I suggest using PP film capacitors for coupling, or PTFE if you want to spend a few dollars more.
Add add individual bias potentiometers to the output stage, and possibly investigate Dave's EFB, to better match the outputs. I also suggest a 10 Ω resistor to allow you to accurately measure the current.
Add grid and screen stopper resistors for anti-arcing and anti-oscillation.
Add a Zobel to the input stage to ensure RF cannot get into the amplifier and cause oscillation.
Replace the screen supply, which is a dropping resistor, with a voltage regulator of some sort. A string of lower-voltage Zeners will reduce the noise of a Zener and improve stability. Do not exceed the published screen limits.
Replace all of the carbon composite resistors, which will have drifted, with 1% metal film.
Replace all of the ceramic capacitors, which are microphonic, hygroscopic, and have other defects such as capacitance varying with DC bias, with PP film.
Replace the sockets.
The 6X4 (like the 6X5) is a bad, bad, tube. It tends to fail as a short which rapidly burns out the power transformer. (A fuse won't help as it takes too long to blow, and the power transformers used in these units are not very robust.) This problem is infamous in the tube radio world. Many circuits overdrive the 6X4 by exceeding the maximum current ratings. (In brief, larger capacitors shorten the conduction angle, i.e. the time in which the filter capacitor charges. The 6X4 cathode can only handle about 40 uF, and even that is pushing it.) I would switch to solid-state with a delay, but another rectifier tube would be fine. Some research on the 6X4 will confirm the widespread problems of blown power transformers.
These changes will give you a nice headphone amplifier which is stable and reliable.Add add individual bias potentiometers to the output stage, and possibly investigate Dave's EFB, to better match the outputs. I also suggest a 10 Ω resistor to allow you to accurately measure the current.
Add grid and screen stopper resistors for anti-arcing and anti-oscillation.
Add a Zobel to the input stage to ensure RF cannot get into the amplifier and cause oscillation.
Replace the screen supply, which is a dropping resistor, with a voltage regulator of some sort. A string of lower-voltage Zeners will reduce the noise of a Zener and improve stability. Do not exceed the published screen limits.
Replace all of the carbon composite resistors, which will have drifted, with 1% metal film.
Replace all of the ceramic capacitors, which are microphonic, hygroscopic, and have other defects such as capacitance varying with DC bias, with PP film.
Replace the sockets.
The 6X4 (like the 6X5) is a bad, bad, tube. It tends to fail as a short which rapidly burns out the power transformer. (A fuse won't help as it takes too long to blow, and the power transformers used in these units are not very robust.) This problem is infamous in the tube radio world. Many circuits overdrive the 6X4 by exceeding the maximum current ratings. (In brief, larger capacitors shorten the conduction angle, i.e. the time in which the filter capacitor charges. The 6X4 cathode can only handle about 40 uF, and even that is pushing it.) I would switch to solid-state with a delay, but another rectifier tube would be fine. Some research on the 6X4 will confirm the widespread problems of blown power transformers.
