Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on August 28, 2019, 07:42:22 pm
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I bought an old Silvertone a while back and started making some changes... then life got in the way. I finally got around to finishing the wiring but now the amp sounds really bad.
I replaced the can filter cap with 3 discrete caps of similar values. I routed the last cap as suggested by sluckey. I also took the filament center-tap and raised its potential by grounding at the cathode of the power tubes (above the 270R cathode resistor). Other than that, I have not made any changes to the circuit (I talked about making some changes, but I haven't done any of those).
Now that amp is VERY distorted. The HT voltages are a little high compared to the schematic but not horrible considering the line voltage is probably higher - 368 vs 350, 350 vs 330 and 280 vs 260. All other voltages are similarly high except one is way out of whack... measuring the cathode pin 8 on one 6V6 is 57 volts and the other is 18 volts. They use a common cathode resistor so I was surprised to see two different voltages. I'm guess this is the problem. Bad tube or something else? I don't have any other 6V6s around to test at the moment.
Here is what it sounds like.
https://youtu.be/TLAgo65w0Mc (https://youtu.be/TLAgo65w0Mc)
Thanks for your help.
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measuring the cathode pin 8 on one 6V6 is 57 volts and the other is 18 volts. They use a common cathode resistor so I was surprised to see two different voltages. I'm guess this is the problem.
I guess so too. How can they be different voltages? They are supposed to be tied together. Apparently they are not. Look for a bad solder joint or broken wire.
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Look for a bad solder joint or broken wire.
That was one problem. I couldn't see that the lead had broken off and the wire was being held on by melted insulation.
However, it still sounded terrible once I rewired the cathode. Then I swapped speakers... aha! Its the speaker. Which is strange because I just took it out of the box tonight for the first time. I will be contacting the company. Hopefully they won't care that I bought it back in January.
Another issue I just noticed... measuring across the heater wires (one yellow lead to the other) I am only getting like 5.1 Volts AC. Could this be causing the flubby bass issue or do you think that has to do with the Phase Inverter Grid Stopper (or lack thereof).
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I would expect 6.3v on the filament string. Unplug all tubes. What do you measure between the yellow wires now?
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I would expect 6.3v on the filament string. Unplug all tubes. What do you measure between the yellow wires now?
Voltage is 5.6v from yellow to yellow. I have grounded the heater center tap (two enameled wires) at the cathode of the 6V6 tubes to raise the potential. Would that cause the lower voltage readings, or do you think I may need to order a new transformer for the heaters? If I recall someone on this board posted they had to get a separate one for their 1482.
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I have grounded the heater center tap
just tape off the CT and use 2 100ohm to create an artificial CT
but while it's down n discharged ohm between each yellow n ct for grins
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That was me.
5VAC loaded should work for 6V heaters. But maybe that's not the point. 5V loaded raises the question that something is wrong. I'd try disconnecting the heater secondary, and hooking up a good outside supply to test.
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but while it's down n discharged ohm between each yellow n ct for grins
0 ohms between each yellow lead and center tap. 263 ohms between yellow lead and chassis ground, which makes sense because I have the CT going through the 270R cathode resistor... it's probably 263 ohms.
I'd try disconnecting the heater secondary, and hooking up a good outside supply to test.
Unfortunately, I don't have any spare power transformers any more. I moved a couple of months ago and stuff had to go. Might be just as cost effective to buy a filament transformer. I think AES had them for $12. I need to get another speaker jack plate which is $0.85 but I was holding off because shipping was $5.00.
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Shipping charges are a major bugaboo! If you have another amp, maybe that'l do to borrow a filament supply? Otherwise, hard to test w/o test equipment, bench supplies, etc. A variac would work for this and other purposes.
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The filament transformer arrived today. I got a 4A transformer. 2*6V6 (0.9) + 2*12AX7 (0.6) + 1*6AU6 (0.3) + 1*6X4 (0.6) = 2.4A so I figured better to go bigger instead of getting a 2A transformer.
Now I'm trying to figure out where/how to mount this thing. Its a little bigger than I expected. I'm thinking either mounting on top of the chassis next to the power transformer, directly above the filter caps. Or I could try to mount it below(ish) the power transformer by using an extra long screw through one corner of the PT, then drilling a new hole and using a standoff.
Check out the pictures to get an idea of what I mean. I'm open to other suggestions.
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Before you mount it anywhere, hook it up and see if that solves any problems.
Then look hard at the left side of the chassis (as seen in your pics).
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I just installed the Phase Inverter Grid Stopper and much of the flubby-ness has been tightened up.
The amp doesn't sound half bad now, but it is not very bright at all, even with the tone knob turned all the way up. EDIT: I just discovered something... the speaker I am using is VERY directional. I had the speaker so that the side was roughly even with my seating position and it was pointing behind me. As soon as I rotated the cabinet 90 degrees so that it pointed AT me the highs came back. Wow! I don't think I have ever experienced THAT directional a guitar speaker.
I'm still getting roughly 5 VAC on the heaters. After listening to the speaker head-on (see edit above), maybe I don't need to worry about the heaters. The challenge with hooking up the new transformer is the leads for the heaters are tightly wound and pretty short. Hopefully I can disconnect them without too much trouble.
Then look hard at the left side of the chassis (as seen in your pics).
Are you suggesting you see something? All other voltages are pretty close to those listed on the schematic.
Here's a question... if only one half of the CT was grounded, would that impact the VAC? I'm pretty sure I was able to get both leads cleaned of enamel and soldered properly, but you never know.
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I'm suggesting possibly mounting the filament PT on the left side of the chassis.
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wind another secondary on the existing PT?
if the bobbin window has enough room to push some 20AWG magnet wire through it. does require removal and disassembly of the PT. probably more work than you'd want to take on.
--pete
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Here's a thought... could I just run 5V6 tubes instead of 6V6? Not sure where I can get a matched pair of 5V6 tubes though. Not finding them on Ebay.
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Here's a thought... could I just run 5V6 tubes instead of 6V6? Not sure where I can get a matched pair of 5V6 tubes though. Not finding them on Ebay.
https://www.tubesandmore.com/products/5v6gt-tetrode-beam-power - 33 in stock. they may match for a fee - ask.
what of the preamp tubes? i'd run with the aux. fil. transformer or wind a new 6.3V secondary on the existing PT if there's room.
--pete
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Here's a thought... could I just run 5V6 tubes instead of 6V6?
I would not go down that rabbit hole!
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Message sent to AES about matching.
I did some searching and a number of people have said the low heater voltage isn't as big a problem for the preamp tubes but that the low voltage could cause "cathode poisoning" in the power tubes. I suppose I may be worrying about nothing. Would a transformer change from 6.3 VAC to 5.1 VAC over time? I believe the tubes are all original in this amp (Silvertone labeled). Maybe it has been this way for 55 years and I really shouldn't worry about it.
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I would not go down that rabbit hole!
If I want to mount the filament transformer there aren't a lot of options. It doesn't fit very well on the left side of the chassis as Sluckey suggests... there isn't enough room between the edge of the chassis and the power transformer. One thing I thought of is mounting it to the front panel of the chassis below the Tremolo and Strength pots. The problem from an aesthetic perspective is that I would have to drill a hole into the Sears | Silvertone logo, which would be a shame.
Here's an idea... could I attach it using something like liquid nails? As long as I ran a ground wire from the transformer frame to ground would there be any downside other than if I wanted to remove it someday?
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Here's an idea... could I attach it using something like liquid nails? As long as I ran a ground wire from the transformer frame to ground would there be any downside other than if I wanted to remove it someday?
I'm thinking this may be the way to go... but instead of permanently mounting the transformer to the chassis, I will attach two bolts. Then I can mount the transformer with kep nuts as if they were coming through. J-B Weld says they have a strength of 2424 PSI. The surface area of a screw head pretty small but as long as I don't torque the screw too much I'm thinking it would have enough strength.
What do you think?
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JB weld might work if you never plan to move the amp around and think it will never get banged or bumped. If it does get banged or bumped, that tranny is likely to come loose and go thrashing about until you arrive at your destination. That thought makes me cringe.
Will it fit within the square indicated below if you relocate that bundle of green caps or replace them with modern caps of smaller size?
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You have the transformer. Just temporarily connect it to the filament circuit. Of course you must disconnect the yellow leads of the original PT. Does this solve your distorted issue? If not, you may not even need the filament transformer. Very easy to try. Worry about physically mounting later.
I kinda like the JB Weld idea. I've had some good results with that stuff. I'd want a quarter size glob to hold a 8-32 nut. I'd want a nut that's about 1/4" thick too. Maybe get a 1/2" long 8-32 standoff and cut in half.
Cathode poisoning is a myth when dealing with receiving tubes. That applies to really high power tubes that operate with very high voltages, such as 50KW radio transmitters.
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Does this solve your distorted issue?
Actually, the amp isn't distorted any more. The Phase Inverter Grid Stopper seemed to have fixed that. Should I just leave well enough alone? I was worrying about the 5VAC on the heaters, but maybe its not really an issue.
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I'd just button it up. It's a budget amp.
BTW, what meter did you measure that 5VAC with? Do you have another meter you could use for a comparison?
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Radio Shack meter from the early 90's. That's my only meter. One of these days I will get a Fluke.
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Radio Shack meter from the early 90's. That's my only meter. One of these days I will get a Fluke.
measure that new hammond fil. transformer - what do you get? it may be the meter reading low on that particular range.
--pete
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New transformer measures 6.6VAC unloaded. It don't think its my meter.
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Had this idea last night while working on mine. Why not drill the rivets out of the Silvertone badge and move it to somewhere else on the chassis. Then you can mount your trans where it was without messing up the plate. Pretty simple to remove with small drill bit and a punch.
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New transformer measures 6.6VAC unloaded. It don't think its my meter.
This is not science. You cannot reason your way to accuracy for test equipment. An unloaded filament tranny could be expected to read 7.1VAC, which is 0.5V more than your meter reads for the new tranny.
Also, your prior reading of 5.6V + 0.5V = 6.1V which is within spec for a loaded tranny. We have determined nothing. The accuracy of your meter is still in doubt.
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Had this idea last night while working on mine. Why not drill the rivets out of the Silvertone badge and move it to somewhere else on the chassis. Then you can mount your trans where it was without messing up the plate. Pretty simple to remove with small drill bit and a punch.
No rivets on my Silvertone badge... its silkscreened on the front panel.
I took it out to play with a band last night and it was actually pretty quiet (no buzz, hum) but it is VERY dark. Even with the tone control all the way up. Is this typical for this amp? Or could this be the speaker (WGS ET65) or could this be a symptom of the low heater voltage?
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This is not science. You cannot reason your way to accuracy for test equipment. An unloaded filament tranny could be expected to read 7.1VAC, which is 0.5V more than your meter reads for the new tranny.
Also, your prior reading of 5.6V + 0.5V = 6.1V which is within spec for a loaded tranny. We have determined nothing. The accuracy of your meter is still in doubt.
Someone needs to send me a calibrated Fluke so I can determine if my RS meter works. :-)
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You've already proven that your meter works.
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VERY dark
start with a speaker, to me the schematic seems to be "bright"
tack a 470pF from top of vol pot to wiper n see if it moves you in the right direction
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Some good info about dark in this thread...
http://el34world.com/Forum/index.php?topic=24009.msg258025#msg258025
In fact, there's a lot of info about this amp. Just search this forum for "Silvertone 1482".
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Upon request, attached is a PDF version of my modded Silvertone 1482 schematic (which appears in ExpressSCH in the thread posted by sluckey). The purpose of the mods was to brighten the input stages, and "improve" PS filtering for an overall less grungy tone. It still retains its bargain basement flavor; I consider this project a success.
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Had this idea last night while working on mine. Why not drill the rivets out of the Silvertone badge and move it to somewhere else on the chassis. Then you can mount your trans where it was without messing up the plate. Pretty simple to remove with small drill bit and a punch.
No rivets on my Silvertone badge... its silkscreened on the front panel.
I took it out to play with a band last night and it was actually pretty quiet (no buzz, hum) but it is VERY dark. Even with the tone control all the way up. Is this typical for this amp? Or could this be the speaker (WGS ET65) or could this be a symptom of the low heater voltage?
OK got ya, I thought you were referring to the other badge that you showed in your picture under the trans.
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Seems to me you could just rotate the filament transformer 90° as seen in the pic 1blueheron just posted. I still suggest you temporarily hook it up to see if you gain anything before drilling any holes.
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I still suggest you temporarily hook it up to see if you gain anything before drilling any holes.
I'm going to do that tonight (hopefully).
Seems to me you could just rotate the filament transformer 90° as seen in the pic 1blueheron just posted.
You can't tell from the picture, but under the transformer 90° are the screws used for mounting the tag strips that hold the filter caps. If I need to mount the transformer, would there be any issue with mounting it as shown in the picture above or is that only a concern with output transformers?
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The transformer will be fine mounted as shown in the pic.
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So I hooked up the the new filament transformer tonight... it measures basically 6.1 VAC loaded. It seems to me that the amp has much more life to it. There are more highs... still fairly dark, but definitely more. Also, the microphone channel has much more gain. So, I think there was something to the low filament voltages and the sound.
In addition, I swapped in new tubes. I think that also helped. I have a new speaker coming so I will try that before I start making more circuit changes.
Now I just gotta figure out how to mount this transformer without accidentally drilling through other wires. Maybe now IS the time to rewire all the heaters.
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Upon request note the chnages I made to the stock 1482:
1. 56K grid stoppers instead of 68K: Re Ch2 these are technically not grid stoppers; they form a voltage divider with the 1M grid leak resistor. This provides slightly more signal voltage than the usual 68K.
2. 1.5K cathode resistor on the instrument channel instead of 2.2K. I wanted to keep Ch1 more true to stock; mod Ch2 to be more Fender-like
3. 104K plate resistor on the instrument channel instead of 330K. same as 2.
4. 280R cathode resistor on the power tubes instead of 270R. Don't recall why but nor a significant difference.
5. 72uF cathode bypass cap instead of 25uF. Experimented with different values while playing guitar.
6. Bypass caps on cathodes preamp and stage of of PI. See below
Here are my notes from 2007: This amp was pretty good stock. As modded it has great tone and sustains forever. Tone is more like a Marshal, definitely not Fender. The sustain however can make the bass muddy when notes overlap, so it’s necessary to pay attention to damping and muting strings. It needs its tone control turned way up. It’s very sensitive to the guitar’s volume & tone controls. Not every setting sounds good. But when you get it balanced, which is easy enough, it’s wonderful. Produces vintage rock tone by the gallon.
Doesn’t like my Strat too much. Les Paul sounds good. The sustain really shines for slide. Ibanez Talman with 3 Kent Armstrong lipstick pickups, tuned to open G, sounds dazzling for slide. (That guitar is gone replaced with a Gibson L6S.)
It was necessary to remove the rear speaker cover. Otherwise tone was muffled, with too much baffle board distortion. Before the last set of mods, it sounded fine with the rear cover on! (The rear baffle board is now back on)
The stock 2-piece Masonite baffle board was replaced with 1/4" plywood and a full 12" cutout. A thicker baffle would not have allowed clearance for the speaker against the rear cover, which is permanently removed now anyway. Joints in the paper board cabinet were re-glued. Rubber or foam a/c insulation was added around the cab near the amp chassis to keep it from rattling against the cab. Most of the cabinet rattling is now gone. This is a cheap amp and I’m gonna keep it that way. Someday I’d like to clone a scratch build with a proper layout and grounding scheme.
Comments on the mods:
Ch 1 grid stop resistor 10K: decouples guitar chord from tube’s grid. Nicer highs.
Ch 2 input resistors reduced to 56K (carbon comp) from 68K: a bit more gain. Lower values resulted in too much gain.
Cathode Bypass resistors on all preamp stages. Cleaner highs. Amp less muddy. Values from 1uf to 5uf seemed to make little difference from one another.
Ch 2 Vol pot bypass cap. Doesn’t seem to make much difference.
No treble bypass caps on attenuating resistors which follow the volume pots. A range of values on a decade box from 500pf down to 47pf were auditioned. There was no detectable change in tone!
Power tube cathode resistor bypass cap increased to 50uf from 24uf. Beefier tone.
Filter choke replaces 2.2K dropping resistor. Relaxed tone.
27K dropping resistor in the B+ line reduced to 22K. The actual stock resistor read 34K. Preamp plate voltages are now closer to spec. Spec calls for 3 watt. Stock resistor was ½ watt. That’s what I used, because it was on hand.
Diode to last filter stage. Increases transient response. Makes bass notes seem more powerful.
.22uf bypass cap across last filter cap. Reduces hiss. Generally improves “tonality”. A range of values on a decade box were auditioned from .047 up to .22uf. .22uf sounded best to me.
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I have a new speaker coming so I will try that before I start making more circuit changes.
I hooked up the WGS G12C today... so much better. Now there is quite a bit more high end than with the ET65.
I decided to measure each resistor and compare to the value identified in the schematic... maybe that was a bad idea. They are all over the place. I'm trying to decide if I should keep the mojo of an amp with drifted 60 year old resistors, or should I try to bring it closer to spec.
There is at least one resistor in the tremolo section that I can get no reading when I measure across it. I don't have a footswitch handy so I haven't even tried it to see if it works.
Any thoughts about replacing the resistors?
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There is at least one resistor in the tremolo section that I can get no reading when I measure across it. I don't have a footswitch handy so I haven't even tried it to see if it works.
Well, I'd replace that resistor. You don't need a footswitch for the trem to work.
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There is at least one resistor in the tremolo section that I can get no reading when I measure across it. I don't have a footswitch handy so I haven't even tried it to see if it works.
Well, I'd replace that resistor. You don't need a footswitch for the trem to work.
I just checked and the resistor I cannot read is a 2.2M going to ground. Could it be that the value is to high for my meter?
And I just remeasured, maybe the resistors aren't as bad as I thought at first. The voltage divider/grid stoppers on the instrument inputs are measuring 87K and 80K (supposed to be 68K) which may be contributing to the low-gain sort of sound in that channel.
The resistors going from the volume pots to the grid of the phase inverter are 289K from mic, 329K from instrument. Not sure if it matters they are 15% different. These seem to change depending upon where the volume pot is set so they were wildly different when I first measured.
There is one resistor that is supposed to measure 330K in the tremolo going from G2 to ground (pin 6) that measures 198K... that is about 30% off.
Other than those, I think they are all close to spec. I was thinking in terms of numbers, not percentages.
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Your meter should be able to measure a 2.2M resistor. Grab a new resistor and see.
You really can't accurately measure resistors in circuit. You need to disconnect one lead to measure accurately.
That 330K that only reads 198K. It's rare for a carbon comp resistor to decrease in value. They almost always increase in value. That .5µF cap in parallel could be leaky, causing a lower reading. Once again, disconnect one end of the resistor and measure to be sure. My money is on a leaky cap.
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Diode to last filter stage. Increases transient response. Makes bass notes seem more powerful.
Can I use any diode here? I think I have some UF4007.
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You need to disconnect one lead to measure accurately.
Whoever wired up this amp was pretty tight with the wire wraps and liberal with the solder. Makes disconnecting one end pretty difficult. Any harm in leaving it.
My money is on a leaky cap.
Stupid question... what exactly is a leaky cap. I guess I always thought it was actually leaking oil or whatever is in them. I'm guessing I'm wrong. And what effect does having a leaky cap have on a circuit.
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Whoever wired up this amp was pretty tight with the wire wraps and liberal with the solder. Makes disconnecting one end pretty difficult. Any harm in leaving it.
Yes, even Sears made good mechanical connections before adding solder back in the day. You need more tools to properly remove a component than was used to install it. I have a drawer full of picks, needles, dedicated desoldering tools, etc. Plus I have an expensive vacuum/desoldering station. It's not easy. Many times its just easier and definitely more cost effective to snip the leads close to the component body then proceed to carefully remove the individual leads to make way to install a new component. Many jack-leg techs would skip this last part and just solder the new component to the old leads. More pride in the $$$ than in their workmanship.
what exactly is a leaky cap.
Not talking about literally spilling it's guts. Most often when someone talks about a leaky cap, they are talking about leaking dc voltage. A perfect cap is just two electrically conductive plates separated by a thin insulator. Heck, I made a cap with a 24x24 piece of window glass, two pieces of aluminum foil, and a bit of duct tape for a science fair project. (see attached pic). Anyhow, that perfect cap with perfect insulation between the two plates is said to have zero leakage. IOW, you could charge the cap to 100V, lay it on your workbench, and the cap would hold that 100V charge forever. But in the real world, the insulation between plates is not a perfect insulator. The insulation will begin to slowly break down with age. (This is very common with old paper/foil caps from the '60s.)The insulation rarely becomes a perfect conductor. It usually just begins to have some high value resistance. This resistance will now allow dc current to pass through the cap. We call this "leaking". You can put a 100V charge on this leaky cap, put it on your workbench, and the 100V charge will slowly 'leak' down to zero. The less resistance the insulator has, the faster the cap will leak. The resistance of the insulation looks just like a resistor in parallel with the cap. It will discharge the cap just as if you placed a resistor across the cap.
The effects of a leaky cap depend on the circuit the cap is used in. In the case of that .5 cap across that 330K resistor, you may notice that that 330K and another 330K resistor right above it form a voltage divider to put a certain voltage on G2. The .5 cap is just used as a filter cap, just like the electrolytics in the power supply. But let's suppose that .5 cap is leaky and it actually has a resistance of 500KΩ. This 500K is parallel with a 330K resistor. Do the math and you see that the total resistance is now 198K. The most noticable effect this will have in this circuit is that the voltage at G2 will be lower than you expect.
And a leaky coupling cap between the plate of one tube and the grid of the next tube will have different symptoms. The plate voltage that leaks thru the cap will appear on the grid of the next tube and upset the biasing for that next tube. If it happens to be a power tube, you may even see red plating. And if there is a volume/tone pot involved the leaky cap will put dc voltage on them and likely make them sound scratchy as they are turned.
If you're still with me at this point, here's the bottom line... Your 330K carbon comp resistor only measured 198K in circuit. Why? A CC resistor usually increases resistance with age, so it's not likely that is the problem, so look to see what is in parallel with the resistor. Ah ha! It's an old .5µF cap, probably paper/foil. That cap is much more likely to be leaky than the CC resistor value decreasing. Hence, my money is on the leaky cap.
I would disconnect the cap and check it for a resistance reading. A good cap will initially 'kick' the meter, then settle down to infinity ohms. A leaky cap will measure some (usually high) resistance. Replace if so. BTW, while the cap is disconnected measure the 330K resistor again. I bet it now reads 330K or higher. Replace if out of tolerance.
And of course, this cap may be fine and the resistor really has decreased in value. But you gotta unsolder some leads to ever know. My money is still on the leaky cap. :icon_biggrin:
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As always, thanks for the clear explanation. It makes sense now. I will have to play with it this weekend.
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> Could it be that the value is to high for my meter?
I can't see your meter from here. Does it have any specs?
A lot of popular-price DMMs only go to 1.99Meg. NOBODY (except tube-amp nuts) uses resistors that large. (Also in implies a test-current of 1uA, which is comparable to leakage in a low-price meter.)
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> Could it be that the value is to high for my meter?
I can't see your meter from here. Does it have any specs?
A lot of popular-price DMMs only go to 1.99Meg. NOBODY (except tube-amp nuts) uses resistors that large. (Also in implies a test-current of 1uA, which is comparable to leakage in a low-price meter.)
Its a Radio Shack from the 90's. There is a Fluke 77-IV for $140 listed locally. Maybe I should invest.
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> Its a Radio Shack from the 90's
Radio Shack stuff has long production runs and the Owner Manual may be on their site. You need a model number.
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Heck, I made a cap with a 24x24 piece of window glass, two pieces of aluminum foil, and a bit of duct tape for a science fair project. (see attached pic). Anyhow, that perfect cap with perfect insulation between the two plates is said to have zero leakage. :icon_biggrin:
You've convinced me, but your Harry Potter impersonation in the pic doesn't appear to have your lady friend convinced... :l2: Just joking, very good explanation and very helpful.
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I wish! That's Miss Mallory, my 12th grade homeroom teacher. Every guy in high school wished she would stand next to them! Heck, there were guys in my class older than she was! Lot of older drop outs came back to school in '66 when the draft rules changed! :l2:
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That's Miss Mallory, my 12th grade homeroom teacher...
I hear she was inspired by your project, stole your idea and went on to launch a successful company producing capacitors.
At least that is the story I'm going with.
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I have the filament transformer installed now and it is an improvement. Things seem more lively than they did before.
While I was in there I swapped the bypass cap on the 6V6 cathode from 25uF to 72uF as recommended by jjasilli. To be honest I'm not sure I can really hear the difference. From what I gather this should extend the bass response but the entire amp is pretty bass-heavy right now so maybe that's not a good thing. I need to get one of those decade boxes so I can switch between caps easily.
I have a couple of questions that fall into "is that what that does" category...
I have been playing with both the microphone and the instrument channels. I am finding the microphone channel to have much higher gain than the instrument channel. The instrument channel sounds downright dull and dark comparatively. Looking at the schematic, there really isn't very much difference between them except that the instrument channel has a 68K resistor in series going to the grid. Admittedly, my resistors have drifted to 89K (if I recall correctly) so maybe this is a good explanation why things seem dull. Is the 68K resistor removing that much gain?
I also noticed that the Silvertone has these resistors BEFORE the 1M grid leak resistor. Fender and Marshall seem to have them after the grid leak resistor. Is this why jjasilli calls it a voltage divider as opposed to grid stopper?
Also, jjasilli replaced his with 56K resistors. Any issue with having one resistor 68K and one 56K so I can have options?
Another question while I'm looking at the preamp. jjasilli installed a 10K grid stopper (after the 1M grid leak) on the mic channel... I'm guessing this will reduce the gain some but not as much as the 56/68K resistors but it will offers some protection from radio frequencies.
I plan on replacing the input jacks on the amp with jacks that will ground when nothing is plugged in. Since all of these resistors are attached to the input jacks (including the pre-amp cathode resistors) I figure now is a good time to ask so I don't have to repeatedly solder and unsolder.
As always, I appreciate everyone's replies.
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I have been playing with both the microphone and the instrument channels. I am finding the microphone channel to have much higher gain than the instrument channel. The instrument channel sounds downright dull and dark comparatively.
I'm surprised. I don't see anything on that schematic to explain that. If the resistors measure the same between the channels then maybe one triode is hotter than the other.
The 68K resistors in the instrument channel do form a voltage divider with the 1M, so the instrument triode would only get 93.2% of your guitar signal, while the mic triode gets 100% of the guitar signal. But I doubt that would cause the mic channel to have "much higher gain", but it will have a little more gain.
For good flexibility I would put a grid stopper on the mic channel and put the 1M on the jack. Then use switchcraft 12A jacks wired like the standard Fender high/low jacks. This can be confusing if you've never wired these jacks. Hoffman has a good picture of exactly how to do this. I have an explanation for the ho/lo jacks in my amp scrapbook.
I would keep it simple and use three 68Ks and two 1Ms and three Switchcraft 12A jacks.
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Some of the things in the last 2 Replies are subjective. Other things could depend on the tubes used, their quality & condition.
To reiterate, the darkness of this amp starts at the input stages. This is a prime example of the maxim that the input stage sets the tone for the whole amp. I found that if you want a brighter tone for this amp you have to start there. So, if you want a brighter tone, then the first step is to brighten the tone at the input stage(s). Everything else is secondary or tertiary to that.
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If the resistors measure the same between the channels then maybe one triode is hotter than the other.
I have swapped several tubes in that position. The original tube, a Sylvania 7025 and a Telefunken 12AX7. All seem to have the same effect. I found this grid stopper calculator (https://www.ampbooks.com/mobile/amplifier-calculators/grid-stopper/). If I plug in the different values I get the following:
No grid stopper (0R): 0.97 (-0.26 dB)
68K grid stopper: 0.82 (-1.70 dB)
89K grid stopper: 0.77 (-2.27 dB)
Another thing that could be contributing... even though I have the volume pots set the same, it doesn't mean they measure they same. So perhaps one is actually turned up louder.
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I just checked and the resistor I cannot read is a 2.2M going to ground. Could it be that the value is to high for my meter?
I cannot find a manual online but I did find a Radio Shack Catalog from 1994 (http://www.radioshackcatalogs.com/html/1994/hr115.html). According to the catalog the meter, Micronata 22-184, has a max resistance value of 2M. So there it could be that my meter won't read that resistor. Doesn't matter, I have one on the way. I could also try string two 1M resistors in series.
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I have made the following changes so far:
- Replaced the input jacks with short jacks (12A) and wired more like a Fender with a Hi and Lo input on the instrument channel. I used 56K resistors for the instrument channel and a 10K grid stopper wired to the tube socket for the Mic channel. This changed the overall gain (for lack of a better term) for the three inputs so they all sound different.
- Added a 2.2uF cathode bypass cap to the Mic channel. Seemed to raise the gain, but tighten up the bass. Still a little tubby for me so I may play with a smaller bypass cap or change the coupling cap. Not sure what would be the "better" approach.
- Replaced the Instrument plate resistor with 100K, cathode resistor with 1.5K and 2.2uf bypass. This lowered the overall gain considerably but still easy to overdrive. Bass is much tighter than Mic channel.
- Added a 470K Phase Inverter Grid Stopper. This was the first change I made. Cleaned up the flubby bass a bit.
Things I tried but didn't like:
- 2.2uF bypass cap on the preamp stage of the PI. This raised the gain a bit. Already have plenty of gain so I took it out.
- 75uF bypass cap on the power tubes. This added considerable bass over the default 25uF. Playing with other values now. I'm leaning towards 10uF.
Now for the questions:
- I want to try adding the diode to the last filter stage. Jjasilli listed 1N4005, but I have a bunch of UF4007... can I use what I have?
- Is there a reason to use electrolytic cathode bypass caps in the power tube? I have some small film caps rated for 100V that will fit just fine.
- Thoughts on lowering the mic channel cathode bypass cap vs lowering the coupling cap?
Overall, this amp is really coming together. This is the first time that I have really played with swapping in values... playing, then swapping again. It has been enlightening.
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I want to try adding the diode to the last filter stage. Jjasilli listed 1N4005, but I have a bunch of UF4007... can I use what I have?
yes
Is there a reason to use electrolytic cathode bypass caps in the power tube? I have some small film caps rated for 100V that will fit just fine.
E-caps are typically used when you need a cap value over 1µF. But if you like the small value film caps then use them.
Thoughts on lowering the mic channel cathode bypass cap vs lowering the coupling cap?
Either will lower the bass. Try and see what sounds good to you.
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> 75uF bypass cap on the power tubes. This added considerable bass over the default 25uF.
If I am understanding right, that isn't right. The cathode bypass on a push-pull power amp has little effect. Combined with "Why so distorted?", I wonder if one of your power tubes is not working.
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> 75uF bypass cap on the power tubes. This added considerable bass over the default 25uF.
If I am understanding right, that isn't right. The cathode bypass on a push-pull power amp has little effect. Combined with "Why so distorted?", I wonder if one of your power tubes is not working.
The phase inverter grid stopper and the filament transformer seemed to fix the distorted issue. Since then, the thread has turned into more of a build log. I'm just using my ears... maybe I'm hearing what I want to hear but it really seemed to me that the bass was VERY full with a 75uF cap and less full with a 10uF cap. Still plenty there for me. I should probably try a traditional single coil guitar with it. I have my Stratotone and a Gretsch hollow body with Filtertrons. Both have plenty of bass.
Regardless... right now I'm pretty happy with the sound.
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Now for a layout challenge...
I want to install some screen grid resistors on this amp. It seems like the common method of doing this is to install a 470R resistor between pins 6 and 4. The 1482 is wired point to point so this is going to be more challenging. The B+ wire is going to pin 4 on 6V6A (for lack of a better naming convention). Then there is a wire from Pin 4 going to the tremolo circuit, and another going to pin 4 on 6V6B. Because pin 6 on 6V6A is unused, I can just move all the wires from pin 4 to pin 6 then install the screen grid resistor as described.
However, pin 6 on 6V6B is already being used. I'm trying to think of how best to install the screen grid resistor. Options I can think of are:
- Drill a hole in the chassis and install a turret (insulated) and run the wire from pin 6 on 6V6A to the turret, then from the turret to pin 4 on 6V6B
- Connect the resistor to pin 6 on 6V6A and have it float over the other components. Extend the lead with wire to reach pin 4 on 6V6B
- Connect the resistor to pin 4 on 6V6B and have it flow over the other components. Extend the lead with wire to reach pin 6 on 6V6A
Any other ideas?
Or, should I not even bother with screen grid resistors.
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Pins 1 & 6 are unused by the 6v6 tube. Those lugs can be used for any purpose, even unrelated to the tube. If those lugs are already used, you have a choice to make. I have drilled and used terminal strips.
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Or, should I not even bother with screen grid resistors.
Bingo!
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I thought grid screen resistors were a good thing to have on your power tubes. Can you explain why they are not needed.
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From a technical standpoint, I believe they are a good thing. However, a lot of cheap amps including your 1482 and it's bigger brother the 1484 didn't use them. I owned a brand new 1484 back in the '60s. I had it for about 5 years and never replaced a 6L6. But the little amp that really convinced me that small amps don't really need them is the famous Fender 5E3. My Fender Harvard and Ampeg J12 don't use them.
I don't know if the screen resistors were omitted because of bottom line $$$ or because it was part of the sound. Anyhow, when I built my 5E3 clone I didn't use any screen resistors.
But back to your amp... The reason I said "Bingo" was two fold. One, the original didn't have screen resistor. Two, there's no quick, easy, neat way to do it in that amp. If pins 1 or 6 had been vacant on both sockets, I would have said put'em on the sockets. Quick, easy, and neat. :icon_biggrin:
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Ok... then I will just leave them out. I was thinking they should be there for safety reasons.
I think the only thing left for me to try (for now) is to adding the diode to the power supply and trying a different (smaller) bypass cap on the Mic channel.
I looked at the coupling cap value more closely... Originally, I was thought they were 0.1uF (like a 5E3) and I was thinking of making the Mic channel's smaller to attenuate some of the bass but then I realized it was actually 0.01uF. It isn't as small a plexi coupling cap, but I think it is small enough that going to 0.0047 would probably not be what I want. I think that the bypass cap will do what I want... although I was playing the amp quite a bit yesterday and maybe the bass is fine.
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I just finished installing a 0.22uF bypass cap around the last cap (10uF) of the power supply. This seemed to be a very subtle difference. Thing seemed slightly more focused or clear for lack of a better way of describing it. Again, very subtle and hard to pin down... maybe I wanted to hear a difference, but I'm pretty sure it is there.
Then I added the diode after the last voltage dropping resistor in the power supply. This definitely made a difference. The attack is very fast and percussive now. I think I lost some of the gain, but I'm not sure why that would be. Maybe I just have the volume knob in a different place. Its getting late and my wife complains. :-)
If anyone has a technical reason that can explain why these changes made the difference, please share. I'm liking the sound very much and I'm a little curious why I don't see these things in many circuits... maybe its because I mostly look at circuits from the 50's and 60's and Fender, Marshall and Vox just didn't do these things.
I think I'm done tweaking now. Just need to get the tolex and grill cloth on the new cabinets (yes plural) so I can start showing this amp off. I'm really enjoying the vibe.
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I just realized... I haven't tried the tremolo yet... I will need to do that.
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If your wife is complaining you must be on the right track!
0.22uF bypass cap around the last cap (10uF) of the power supply
the diode after the last voltage dropping resistor in the power supply.
I learned these PS mods from "Inside Tube Amps", by Dan Torres. The first mod is borrowed from hi-fi. Real world caps are imperfect; the larger the cap, the more imperfection. Large electrolytics are particularly "obnoxious". A quality plastic bypass cap helps them do their job. Typically this bypass cap should be 100X smaller than the electrolytic. But I A/B different values while playing the guitar and settled on .22uF for this amp. Attending to the 1st gain stage (last filter stage) gives the most bang for the buck. In hi-fi tube amps, more filter stages might be bypassed.
The diode: When the Power Amp draws more current, causing voltage sag, it can suck current from the wall outlet, or from the preamp. The diode "protects" the preamp, preserving attack in the preamp. This compensates for droop in the Power Amp, which I like. I also like chokes which slow things down due to their hysteresis; and the diode compensates for that too, in the preamp. I consider these my standard PS mods: choke, diode, bypass cap. I think tube rectification (or SS with inductive dropping R) & choke contribute to a blues or vintage rock tone.
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The diode: When the Power Amp draws more current, causing voltage sag, it can suck current from the wall outlet, or from the preamp. The diode "protects" the preamp, preserving attack in the preamp. This compensates for droop in the Power Amp, which I like. I also like chokes which slow things down due to their hysteresis; and the diode compensates for that too, in the preamp. I consider these my standard PS mods: choke, diode, bypass cap. I think tube rectification (or SS with inductive dropping R) & choke contribute to a blues or vintage rock tone.
So what I am perceiving as "less gain" is really "preserving attack" which is why thing seem more clean. Might be interesting to put a switch for this... although I won't do it for this amp. Not a lot of space left.
Where did you mount the choke? Since I already have a filament transformer installed, I'm not sure I have room.
By the way... I took it to band practice yesterday. I was using it as a head because the existing cabinet rattles and the screws to mount the speaker are starting to pull through.
This little beast is pretty loud. I could not turn it up past 2, which is sort of a drag because I like the tones around 4. But, it took pedals really well. When I started it got really farty sounding with an overdrive pedal. It didn't have any of that yesterday.
Now I really want to continue with making a turret board layout and who knows... maybe I will make a head with some switches for the various options.
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The choke is mounted near the PT. The supplemental filament tranny is on the bottom wall.
I often find that 12 - 20W is too loud, depending on size of venue, and vol drums & bass. I built an attenuator, schematic below.
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Now I really want to continue with making a turret board layout and who knows... maybe I will make a head with some switches for the various options.
I made a layout a few years ago. If this looks interesting I will proofread the layout and email the Visio file to you.
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I made a layout a few years ago. If this looks interesting I will proofread the layout and email the Visio file to you.
Of course you did. Just when I thought I might get to do something new. :-D
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I will proofread
Just when I thought I might get to do something new
You can proof it for Steve :icon_biggrin:
course it's a waste of time but hey :laugh:
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I thought you used Visio, but I just looked at your board layout for that BandMaster. Now I'm not sure. Can you confirm? If no Visio then I'll just do a pdf. It will include a full size drill guide so making this board will be easy. Of course you may want to just use it as a guide and do your own layout. This layout is optimized for 1/2W metal film and Xicon 'chicklet' caps but I believe those pretty 1W carbon film resistors you used on the BandMaster will fit also. The board is 3-1/8 x 8.
So, which do you want, Visio file or PDF?
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So, which do you want, Visio file or PDF?
I used to use Visio but now I'm on Mac and no longer have a license. So it will have to be a PDF. That said, I was just looking for a project to learn DIYLC as well as design my own board so not really necessary.
A number of years ago (wow, its been that long) I went through some lengthy discussions on this board about designing a low-wattage amp using a single 12BH7 for the power section. Maybe I will go back to those notes and take a stab at laying out that amp.
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I finally got around to cleaning the pots on my Silvertone... had to buy some Deoxit. They were scratchy but useable. I didn't do a before and after listen so it may be psychological, but I swear there are more highs after I cleaned the pots. Could it be that some of the darkness I was complaining about was all due to dirty pots?
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Maybe you got some in your ears too? :laugh:
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One mod in my 1482 that I use a lot is on my mic channel I installed a Princeton 5F2 tone stack. The 1482 circuit is just naturally dark for what ever reason. This mod adds some nice controllable brightness. Platefire
My Schematic