Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 1blueheron on December 03, 2018, 09:40:02 pm
-
Anyone have any information on this Airline? Google not returning much. Was it made by Valco or Danelectro? Model seems to be missing from the schematic library.
Here is a pic of the model number. Chassis and tubes looks like they are all there. Cabinet is very rough and the speaker cone has some tears. Grill and grill cloth seems ok. Worthy of time, $ and effort to restore? Good candidate to learn basics of repair and restoration on?
-
grill
-
Maybe this...
-
The circuit looks very much like the silvertone 1472/1482 circuit. The inputs, the rectifier and power look different, but everything else looks similar. I'm guessing that is Danelectro made. Maybe a 10 watt amp instead of 15 watt.
-
Can you post a clear pic of the tube chart on the amp cab wall?
-
Maybe this...
Thanks Sluckey, Not sure of the significance of the "a" designation in addition to 8405 but I think it is probably the correct schema. Since the amp is not in my hands I cannot yet confirm.
Can you post a clear pic of the tube chart on the amp cab wall?
Here is a pic and clarification of the tube chart on the amp wall.
-
FYI, there is one more 12ax7 i believe in the upper control panel not shown in that tube layout.
-
that's the same tube lineup in the schematic that slucky linked to.
--pete
-
FYI, there is one more 12ax7 i believe in the upper control panel not shown in that tube layout.
Correct. It is in the trem/control upper chassis unit.
that's the same tube lineup in the schematic that slucky linked to.
--pete
Yes, I believe Slucky has given me the correct schematic, just not sure if there is a subtle difference in the "a" designation.
-
Not the same amp, but the silvertone 1344 and 1344a are the same amp except the "1344a" had a speed control and an intensity control for the tremolo/vibrato. the "1344", just had intensity. Could be something like that.
-
Further investigation says no real difference between the "a" and non a versions.
I closed the deal on the airline and should be picking it up next Sat. Planning a full resto.
Found a Youtube on resto of the 8503.
Also found it is pretty much identical in circuitry to the Danelectro Centurions.
-
Picked up the Airline 8504 and brought it home yesterday. Took it apart today for a thorough cleaning and assessment.
The chassis' both cleaned up nice.
I am not sure the speaker is original to the amp. It is a 15" speaker EIA numbers 328001 fix it as a Utah. Was able to patch the damaged areas in the cone and it appears it will be serviceable. Cutout in the baffle is smaller than 15" but there are no unused mounting holes. Strange and puzzling. Anyone know for sure what size/make speaker was stock in the Airline 8504?
The tolex is pretty worn and the back panel is missing. It was replaced with a homemade panel which was installed lower in the back of the amp. I am guessing this was to accommodate the 15" speaker of which the magnet extends back slightly too far for the original back panel to work. Planning a workaround for this.
I need to replace the 2 wire power cord, then plan to do voltage checks and see what i've got.
The 6V6 tubes are not matched up. One is a Sylvania, the other is an RCA. Sylvania has a halo and they have different plate styles and lengths. I am guessing they should be replaced with a matched set? Will post pictures up shortly of the progress.
-
> a matched set?
Why?
It will play fine, unless tubes are outright sick.
-
> a matched set?
Why?
It will play fine, unless tubes are outright sick.
Well, I just thought it would probably sound better if the pushing and pulling was equal, I'm sure it will work as it is but maybe not sound its best?
-
Here's some before and after the cleanup.
-
More pics..
-
Hoping to get the power cord replaced and the "death cap" removed today.
-
New 3 prong power cord installed.
Removed c14.
Plugged it in and powered it on to do voltage check but started getting smoke from c9,10,11 bundle so I shut it down.
It appears C13 has already been replaced as it is now a 20MFD rather than the 16MFD on the schematic. It looks like it is new as do C12A and C12B.
Can anyone tell me where C6 is? Not seeing it. :w2:
Looks like I will be ordering parts.
-
C6...
-
Thank you Sluckey.
-
All of those Sangamo Type 33 caps are labelled as 600V. If replacing is it necessary to use 600V or is 400V sufficient? I don't see any points where voltages over 400 would be encountered. Am I looking at this wrong?
-
I would use Mallory 150s or Xicons for replacements. Both are small and rated for 630V.
-
Thanks Sluckey. I had those Mallory 150's in the shopping cart when I posted but seeing Doug is closed until next week and I was wondering if something else in my scavenged parts bin would suffice for testing until I could get proper long term replacements.
Do all the caps in the amp need to be +600V rated (such as C15,16,17,18) or can these be of lesser voltage? I tried Google on this and it seems the opinions vary. Some contend unloaded rail voltages can be high before load is applied or when testing with some tubes removed, while others say 400V in the preamp section is adequate and may be fine other places depending on rail voltage. Also arguments about continuous use vs. short term tranisents etc.
In my case, the power supply filter caps are rated at 450V and the highest rail voltage listed is 280V which makes me wonder if OEM 600v was overkill.
I have some 400V Blue Molded caps in proper values on hand and I also have some 6PS 600V Orange Drop caps and Aerovox Brown Dip 600V caps
Is it inadvisable to use 2x .047uf 400V Blue molded caps in series to replace the .02uf 600V caps (C9,10,11)? That would give me .023uf at 800V correct? Or similar combinations of series/ parallel with the 600V orange drops/brown dips to get proper values for other caps?
-
Found another close relative...
https://www.vintageguitar.com/22134/danelectro-viscount/
-
400V coupling caps will work - worst case is 375-ish volts on cold startup startup for a few seconds.
--pete
-
Thanks Pete! That means I can probably get it up and running for voltage checks today!
:icon_biggrin:
-
I postponed voltage checks and re-power due to some concerns about the 6X5.
Here are my concerns:
I have very high mains voltage here. We are typically 125V. From using that dangerous tool called Google, I ran into a mess of posts about how the 6X5 has issues with modern day line voltages. Many recommend replacing it with diodes, converting it to 6X4 or converting to an EZ81.
Is there anything here to be concerned about or is this all just a tempest in a teapot.
My old Eico tone generator suffered a 6X5 failure when I plugged it in. Probably caused by a faulty cap but it blew the 6X5 on power up. Perhaps this makes me over sensitive to this issue.
I Have plenty of empty octal bases I can make a ss replacement with if that is the better/safer way to go without making radical modifications. I realize it will change the sound but since we don't know what it sounds like yet, I'll never know the difference.. :l2:
-
We are typically 125V
The datasheet I looked at says 325Vac cap, or 425vac choke, rms per plate so that is your starting to guess point
-
re 6X5GT. There was a recent thread about 6X5/6X4's --> http://el34world.com/Forum/index.php?topic=23673.0 (http://el34world.com/Forum/index.php?topic=23673.0)
I have an airline with a 6x4. I put some 1N4007's in series with the 6x4 and decided not worry about it anymore. (in series between PT and tube, using unused pin lugs on the sockets --if you have unused lugs ., it looks like danelectro used a few on that octal...)
that is a certainly a danelectro build. notice the "GDR" in the GDR8504 was the montgomery ward identifier they stuck in item#'s to identify the manufacturer. GVC was valco. as for the "A" suffix, that is just a catalog pn# suffix number. I'm not exactly sure how MW decided on the suffix, but the "A" wasn't anything to do with an engineering version (like a 12AX7A, or a 6L6A..) and it wasn't specific to amplifiers. The guitars has that suffix as well. Sometimes the catalog pn# suffix would be different if it was a bundled set., like a guitar might be a 7092A and the case is a 7766A but the bundled guitar and case was 7093AA ?!!? also, for amps, sometimes the catalog pn# would be a "R" or "RTT" for amp + cabs, and there are some combo amps with a "RD" or a "B" suffix (no bundling needed for a combo). "62" is something you see a lot with MW amps and guitars., "62" was the department number.
-
We are typically 125V
The datasheet I looked at says 325Vac cap, or 425vac choke, rms per plate so that is your starting to guess point
Shooter,
Thanks, So I have a cap filter input amp if I am not mistaken.
The schematic references line voltage at 117Vac, and shows rectifier plate voltage as 260Vac. I assume pulling the tubes and checking voltage at pins 3&5 will tell me if I am within the 325 design limit while running it at 125Vac line current, and I assume I will will probably be in the 280-290V range at pins 3&5.
Does this mean I am in the clear and have no reason for concern? My thinking is that if I replace the caps and check all the resistors and everything is good, it should work fine for another 60 years... :w2:
re 6X5GT. There was a recent thread about 6X5/6X4's --> http://el34world.com/Forum/index.php?topic=23673.0 (http://el34world.com/Forum/index.php?topic=23673.0)
I have an airline with a 6x4. I put some 1N4007's in series with the 6x4 and decided not worry about it anymore. (in series between PT and tube, using unused pin lugs on the sockets --if you have unused lugs ., it looks like danelectro used a few on that octal...)
that is a certainly a danelectro build. notice the "GDR" in the GDR8504 was the montgomery ward identifier they stuck in item#'s to identify the manufacturer. GVC was valco. as for the "A" suffix, that is just a catalog pn# suffix number. I'm not exactly sure how MW decided on the suffix, but the "A" wasn't anything to do with an engineering version (like a 12AX7A, or a 6L6A..) and it wasn't specific to amplifiers. The guitars has that suffix as well. Sometimes the catalog pn# suffix would be different if it was a bundled set., like a guitar might be a 7092A and the case is a 7766A but the bundled guitar and case was 7093AA ?!!? also, for amps, sometimes the catalog pn# would be a "R" or "RTT" for amp + cabs, and there are some combo amps with a "RD" or a "B" suffix (no bundling needed for a combo). "62" is something you see a lot with MW amps and guitars., "62" was the department number.
Terminalgs,
Thanks for confirming the model/letter designation and history bit. I got the amp from my BIL. He has an Airline Bobkat guitar that he thinks came with it. Is it possible the "A" was a designation that it was part of a Guitar/amp starter kit in the catalog and sold together? This might explain a few things. I can confirm that everything is accurate on the schematic Sluckey posted.
The diodes you mentioned as having placed in series with the 6X4, they serve just as a safety measure but have no effect as long as the tube is functioning correct?
I ran across mention that it might be possible to use a 0Z4 or 0Z4G as substitute for a 6X5. Full wave rectifier, octal base, similar specs. However it needs 300V to fire up. Not sure if this is correct but might be an interesting solution :dontknow:
-
unloaded will give you kinda sorta, the real test is loaded at Time 0 :icon_biggrin:
you should be fine, for another 60yrs......optimism is a good thing :laugh:
-
> 0Z4 ....it needs 300V to fire up.
NO, that is a maximum it should ever see!!
It conducts at 24V.
It really is not suited for more than 250V car radio application.
Even then, it was rarely used--- maybe a clue?
My suspicion is that WHEN it shorted, it took-out the rest of the power supply with it. A vacuum rectifier has some limiting resistance.
6X5 is not unreliable. Don't let bad experience spook you.
-
OK, moving on. Removed tubes and checked PT voltage last night.
Voltage at wall= 124.9V
Voltage to chassis ground at pins 3&5= 291V
Voltage to heaters 6.9V
In other business, replaced the 4 Sangamo caps in the preamp section. In the process found a connection that appears to have never been soldered before it left the factory. Just stuck in the terminal loose. WOW! Things were not very neat up there. I think they went to above and beyond to reduce component numbers in this amp :l2: Some really rough looking solder joints. Re-flowed all of those and inspected.
Checked resistor values in the preamp section. Most of them have drifted upward a bit. Ie. 68k are reading in the mid to upper 70's but I guess that is normal. Will leave them as is for now.
-
10% would be about 74k. That's within the tolerance. I wouldn't worry about it.
-
Limited time to fiddle last night but did find that the two 68ohm resistors on the FIL side of the PT are considerably off reading 45ohms. Planning to replace those. Also found many of the 330k resistors are outside the +/- 10% range. Worst was R18 2.2K which was toasted swollen, and reading .84K. I replaced it.
Also replaced the .05 sangamo at C8 with a .056 orange drop 6PS
The person who replaced the Filter caps previous to me snipped the originals and soldered the new ends to the old ends and it looked pretty crappy so I removed and soldered that to the lugs properly.
Pretty much at the end of my rope until parts come now.
-
Got a few minutes last night to spend on the amp and as luck would have it when I got home, most of my caps had arrived as well as my new weller soldering station. Woo Hoo.
I have 2 caps left to replace that have not yet arrived. Resistors should arrive before the end of the week.
Now has a F&T 16MFD 475v for C13
C3 is 10uf 50V Illinois
c15&c16 are 400V Goodall. May change later to experiment with sound.
c17&18 are 600V brown drops (might play with these as well)
c8 600V Orange drop 6PS series.
Remainder are Mallory 150's
The 2- 10uf caps will be replaced with Nichicon VX series 10uf 450V when they arrive.
Here is what it looked like after I finished last night.
-
Neat work.
-
Neat work.
Thanks Sluckey.
-
Yeah. My first couple of amps looked like I shoveled the parts in there. Very nice work!
-
Is this indicating the voltage at this node should be 450V or is it indicating the voltage rating of the 16uf cap?
-
I would say the latter? Could be wrong.
-
Cap, node vdc is 280
-
Cap, node vdc is 280
Thanks for confirming. That's what I thought but it is strange no other cap values are listed other than C3 which is pretty obvious, so I wanted to be sure I hadn't missed something.
Also, just to confirm, the Mallory 150 caps are non-polar correct? Everything I read says they are NP but there are lots of dissenting opinions on inner/outer foil and noise. Any truth there or just noise? :l2:
-
read n decide, also, in the achieve section is a thread, "do caps sound different"
the mallorys are NP
http://el34world.com/Forum/index.php?topic=11427.0
-
read n decide, also, in the achieve section is a thread, "do caps sound different"
the mallorys are NP
http://el34world.com/Forum/index.php?topic=11427.0
Thanks for that... Now I have to add checking polarity on all the caps to my todo list.... :sad2:
Well, all the parts have arrived and are now installed! Woo Hoo!
We have been running on generator power here for two days so voltage has been too sketchy to do much testing but it was fine to run a soldering iron.
After all the cap and resistor work was done, I put in all the tubes minus the rectifier and checked to see if all the heaters lit. They did, Then I put in the rectifier and plugged in the guitar. Man is this amp quiet! Too quiet. Absolutely no audio output, not even hiss or hum coming from the speaker. :sad2: :help:
Will try to do a full voltage check tomorrow night and post findings.
-
The problem was a pretty simple fix. I found it tonight while attempting voltage checks. I couldn;t get a reading on voltage between ground and any of the pins.... Then I realized when I installed the 16MFD F&T cap, I moved the CT and cap over one terminal to make them line up better. The terminal they used to be on was the grounded terminal, so I had removed the ground for the CT. :BangHead: installed a jumper and bingo, it is now alive and kickin'.
How it sounds:
It sounds really nice but doesn't have much sparkle. Perhaps its the 15" speaker in it. Can't help but think it is rolling off some of the highs. The trem works nice.
Things I still need to do:
Channel 2 is dead for some reason. Need to get that sorted out.
Try it with a 12" speaker that will fit with back cover on
Recover the cab and do some remaining cosmetic work
Install some rubber feet.
On the second channel issue, what would be required to wire the second channel (other side of the 12AX7) as a preamp boost or overdrive channel? We really have no need for six inputs on 2 channels. Seems like it might be a pretty simple and reversible mod that would make the amp a bit more versatile. Also thinking of using slightly different resistor values for each of the 3 input jacks rather than them being all the same.
-
Channel A and Channel B are identical and very simple. Just compare voltages and resistance values (and caps too) between the two channels. Should be a quick and easy fix. Once you have both channels working try this simple cascade mod. Should give you quite a boost. But fix the dead channel first.
-
Thanks Sluckey, have I ever told you your awesome! :icon_biggrin:
Will keep you posted on outcome.
-
Can you post some hi-rez pics of the preamp chassis?
-
Didn't end up with much time last night but did find the problem with ch 2. and took some photos.
When looking at schematic and photos this AM, I realized I am probably referring to Channel A on schematic as Channel 2, and Channel B on schematic as Channel 1 (backwards). I have always been called backwards, I guess this confirms it. Anyway, I labelled all the photos to match what I am calling channel 1 and channel 2. For clarity, my channel 1 connects to pins 1,2, 3 on the 12AX7 and channel 2 connects to pins 6,7,8. which to me is logical. :icon_biggrin:
-
Hoping to fix the wiring issue with ch. 2 tonight and test. If all is good I will attempt Sluckey's mod.
By performing the mod, will that make the volume control for Channel B like a gain control and the volume for channel A like a MV? It appears I am just feeding Channel B output back into Channel A input correct? I assume this will make channel A "normal" and channel B "High Gain" and when playing in normal mode Channel A, the volume and tone should be turned down on Channel B. Is this correct or doesn't it matter?
-
According to the schematic, channel A connects to pin 7 and channel B connects to pin 2. Since the channels are identical it makes no difference what you call them as long as you know which channel you are talking about. I'm gonna use the channel designations shown on the schematic so I will probably stay confused. :laugh: (Wish you would use the schematic designations too.)
About the mod... The answer is yes to all your questions except it should not matter about turning Channel B vol/tone down when using Channel A. This is a simple/easy mod. If this mod proves to be useful I would replace the three input jacks for Channel B with a single switching jack wired so the jack grounds the B input when not used. Or, wire a simple Fender two jack (HI/LO) input. Probably a good idea for Channel A also.
-
Got it. Thanks. I will from hereafter refer to the channels as designated on the schematic and try to prevent further confusion. :think1:
-
I dug into it last night for a bit. Fixed the issue I was aware of but it did not fix the problem. So that sent me troubleshooting.
Found I only had 90V at one of the plates. The other side was reading 2V. Oops. Whats wrong. Well it appears I connected R27 to pin 8 rather than pin 6. :BangHead: That was dumb. Gotta get better at checking myself against the schematic.
So once I found that, I set about to fix it, but then decided I would like to make the caps and resistors more accessible than what they currently are so each time I change one for experimenting/modifying, my wires don't keep getting shorter from the amp chassis. I don't have any length to spare on the heater wires. So I started to redesign the board that they are mounted on. I almost have a new board finished with some standoffs for all the wire terminations from the amp chassis, the caps, and the resistors. Should have it finished up tonite with any luck and hopefully everything will work right this time and I can move forward.
I guess I am making some good troubleshooting practice for myself.... Learning a good bit in the process.
-
I guess I am making some good troubleshooting practice for myself.... Learning a good bit in the process.
That's good. See how checking tube voltages can lead to a problem? It's really nice to have an identical circuit that's working so you can compare voltages. When you have the amp running properly consider measuring voltages on all tube pins and write them on the schematic for future reference. I even like to write down zero volts.
-
The amp is now working 100%. Put the new board in this AM and both channels worked great. Did the suggested cascade mod and wow. Has some gnarly tones. Still need to get everything labeled and I am going to check all voltages and document for future reference.
I think a new 10 or 12" speaker and a solid pine cabinet would really make this amp sound good. we are getting a bit of non-harmonic distortion from bass notes when its cranked up a little. Might be the metal grill rattling or possibly the speaker has some issues to check into. It is pretty bass heavy. Hers some pics of the new board.
-
Nice job!
Did the suggested cascade mod and wow. Has some gnarly tones.
So, will the mod be useful? I have some ideas to add a bit more gain or tame the gain a bit if you want to experiment. Your new board would make it easy.
-
Yes, The mod is very useful. Thank you. My son says it has some "early Marshall" tones.
I am open to further ideas. I have only 2 input jacks now rather than 6. Need to switch them out to shorting style. Need to fill the unused holes to make it look decent. Was considering making the cascade mod switchable by putting a selector switch in one of the jack holes. Then I can run it as stock real clean, or switch to the mod for high gain. You probably can see that I moved the 68K and 1M resistors from the jacks to the new board to shorten the length of signal exposed. I am going to make the wires from the jack to the board shielded. Once I get a self shorting jack, I am considering changing value on the 1M and adding a capacitor.
In the cascade mode, we are getting some nice breakup but have more than enough volume. If I put in a DPDT 3 position selector switch as mentioned above and an attenuator resistor on the speaker output that engaged only in the high gain mode might tame some volume while still giving good breakup? Or maybe lower the plate voltage on other half to encourage an earlier breakup? I don't know whats your ideas? Enjoying this amp as a test bed for ideas.
-
I am considering changing value on the 1M and adding a capacitor.
Why change the 1M? That's the most common value used on the input jack. I'd let it be. What cap are you thinking about adding?
Switching will be very simple with a DPDT switch. No need to do anything with speaker output. You don't really want speaker signal up there in the preamp chassis.
You should be able to use CH B volume as a gain control and use CH A volume to set the volume level.
-
OK, scratch the change to the 1M.
Does this look like a reasonable set of mods?
Change 68K gridstoppers to 10K and compensate with 200pF caps. Supposed to have lower hiss levels?
Install switchable (S1) cold cathode clipper using 10K resistor on cathode of V1B. Supposed to achieve smoother/earlier overdrive? Will this work or do I need to make additional changes if I do this?
Install switch (S2) to make amp switchable between stock and cascaded mode.
Thoughts? Dumb idea?
-
That will work, but... I don't think hiss will be a problem so I would drop the 200pF caps. I also don't think the 'cold clipper' is needed because this is not a real high gain mod. Cold clippers are usually seen when you have three (or more) gain stages.
Here's what I had in mind...
-
OK,
That looks simple enough.
What should I expect the optional 2.2uf caps do in terms of tone?
Removing the .003 cap on the tone pot will render that tone pot disabled correct? I am not connecting the pot wiper directly to grd?
-
The optional cathode caps will give you a fatter tone and also a bit more gain.
Removing the .003 cap on the tone pot will render that tone pot disabled correct? I am not connecting the pot wiper directly to grd?
Yes and yes. I would rather just disconnect one end of the jumper between the VOL and TONE pots. Easy to reconnect if you think you need it.
You can also try your hiss caps and cold clipper switch in my schematic.
If you try my circuit, don't overlook the diagonal jumper on the switch. It is required for proper operation.
-
What voltage rating is needed for those 2.2uf caps?
-
16 or 25. They will actually only have a couple volts on them.
-
I think I see a problem with your new board. Look at the attached pic. If that screw is used to mount the board to the chassis, there's a problem. Measure resistance from pin 2 to chassis. If it's only 68K then you indeed have a problem.
Easy solution would be to use a nylon screw.
-
How did you clean that chassis, looks great?
-
I think I see a problem with your new board. Look at the attached pic. If that screw is used to mount the board to the chassis, there's a problem. Measure resistance from pin 2 to chassis. If it's only 68K then you indeed have a problem.
Easy solution would be to use a nylon screw.
Sluckey,
The screw you mention does not touch the chassis. In fact the entire board floats on little rubber grommets. I believe the purpose in this was vibration isolation to avoid microphonics? Anyways, that is why I ran ground wires back to the chassis/jacks. The standoffs and board is isolated. Thanks for taking the time to look behind me though. It is much appreciated as I am still making plenty of mistakes.
I am going to check my parts bin tonight and see what I have in the way of low voltage 2.2uf caps. I found a DPDT switch last night, but it is ON_OFF_ON. Will this cause popping or will it work fine? Do I need to order a On-On?
-
How did you clean that chassis, looks great?
Dude,
Thanks,
First I get all the loose mess an dust off with a paint brush. That got all the dog hair, crud, etc off. Then a soft cotton cloth dampened with some water and mild detergent. Then I came back and used some Mothers Aluminum polish to remove the oxidation. That stuff is the bomb. Brings back a mirror like finish and didn't seem to bother the stenciling on the amp. I used a Dremel tool with wire brush to clean up the nuts/shafts on the jacks and pots as they were all showing some oxidation.
-
The screw you mention does not touch the chassis. In fact the entire board floats on little rubber grommets. I believe the purpose in this was vibration isolation to avoid microphonics? Anyways, that is why I ran ground wires back to the chassis/jacks. The standoffs and board is isolated. Thanks for taking the time to look behind me though. It is much appreciated as I am still making plenty of mistakes.
I understand about the grommets acting as a shock absorber, but they look dry and hard to me. But... The screw head is in contact with the terminal strip lug and I bet the other end of the screw and nut is in contact with the chassis. Make the resistance check I asked about. I bet it is 68K.
I am going to check my parts bin tonight and see what I have in the way of low voltage 2.2uf caps. I found a DPDT switch last night, but it is ON_OFF_ON. Will this cause popping or will it work fine? Do I need to order a On-On?
2.2µF is not a magic value. You can try anything from .68µF up to 25µF. The bigger caps will give more bass, maybe even too boomy for some people. And you can use different values for each cathode too. That switch will work. Just remember that the center position will kill all sound.
-
Thanks for the answers. Just ran the check on the screw/chassis ground. It is isolated. Meter registers nothing between Pin 2 and ground. This was with the board in place. If I check the nut to chassis, I've got 69k so it seems everything is correct.
Found some 2.2uf 50V caps. Will be giving that a try. Will try some other values as well just for grins and giggles.
Also going to hook up a 12" speaker and see how that sounds in place of the 15".
-
Thanks for the answers. Just ran the check on the screw/chassis ground. It is isolated. Meter registers nothing between Pin 2 and ground. This was with the board in place. If I check the nut to chassis, I've got 69k so it seems everything is correct.
I don't think we are communicating very well. Pin 2 to ground should read 1M. How can the nut possibly read 69K to chassis? Isn't the nut in contact with the chassis? Can you post a pic that shows the end of the screw and nut on the other side of the chassis?
-
I am sorry, maybe these pictures will explain it better? At no place does the screw or nut contact the chassis. There is rubber between all the parts. Screw goes through grommet which has rubber inside chassis hole, nut screws against outside of grommet and does not touch the chassis. Chassis ground connections for pre-amp are through the indicated black and green wires which connect to the ring terminal of the two input jacks.
-
Screw goes through grommet which has rubber inside chassis hole, nut screws against outside of grommet and does not touch the chassis.
Ding! That was the missing link for me. I got it now. Sorry for my confusion.
I see a couple issues with your board but I'll do a separate post for those. So, have you tested the cascade mod with your new board? Any issues?
-
Screw goes through grommet which has rubber inside chassis hole, nut screws against outside of grommet and does not touch the chassis.
Ding! That was the missing link for me. I got it now. Sorry for my confusion.
I see a couple issues with your board but I'll do a separate post for those. So, have you tested the cascade mod with your new board? Any issues?
I took the preamp apart last night hoping to start on the new mod. Got as far as trying to put my DPDT switch in but realized the switch shaft size is too large to fit in the empty 1/4" jack hole. I really want to avoid doing anything irreversible to the chassis like boring larger holes so I am trying to find/buy a switch that will fit in stock hole.
At that point I took some pics and called it a night.
What has been tested so far is the output of channel b tied into the input of channel a. That worked fine fed from separate input jacks.
Have not lifted tone control, installed cathode caps, relocated the 1M or installed the DPDT switch.
-
You need a mini DPDT switch like Hoffman sells. The switch fits a 1/4" hole so you will need a bushing with 1/4" ID and 3/8" OD. A piece of 1/4" ID copper tubing should work. Look in plumbers supplies. You could even use a piece of 1/4" plastic/rubber fuel line or air hose.
About that screw/nut... That screw is in contact with the terminal lug that will connect through a 68K to the grid of the first triode. If you touch the end of the screw/nut you will get a buzz just as if you had touched the tip of the plug on a guitar cord. That screw/nut is like an antenna and it "may" pick up radiated noise from the transformers, house lighting, radio stations, etc. I still recommend using a nylon screw.
I've put together a drawing that shows my mod ideas and also addresses a couple issues I see with your board. Should be easy to work into your new board. The cascade switch is wired the same as Hoffman's hot switch. If you have any questions, just ask. I'm following this project with interest.
And if you decide to use another plan that's OK too. Still take a look at the attached pdf. It may give you some ideas.
-
You need a mini DPDT switch like Hoffman sells. The switch fits a 1/4" hole so you will need a bushing with 1/4" ID and 3/8" OD. A piece of 1/4" ID copper tubing should work. Look in plumbers supplies. You could even use a piece of 1/4" plastic/rubber fuel line or air hose.
About that screw/nut... That screw is in contact with the terminal lug that will connect through a 68K to the grid of the first triode. If you touch the end of the screw/nut you will get a buzz just as if you had touched the tip of the plug on a guitar cord. That screw/nut is like an antenna and it "may" pick up radiated noise from the transformers, house lighting, radio stations, etc. I still recommend using a nylon screw.
I've put together a drawing that shows my mod ideas and also addresses a couple issues I see with your board. Should be easy to work into your new board. The cascade switch is wired the same as Hoffman's hot switch. If you have any questions, just ask. I'm following this project with interest.
And if you decide to use another plan that's OK too. Still take a look at the attached pdf. It may give you some ideas.
Sluckey,
Your observations and recommendations are greatly appreciated and it all makes sense. Moving the caps to avoid heater wires is a no brainer and I should have seen that.
So last nights progress was minimal. I spent some time digging through my piles to find a mini-toggle. Found 3 so I think I am in good shape. Also found some multi position rotary selector switches, which I assume would work well for this application and fit in the stock 3/8" holes. I looked breifly at what it would take to put one of these in the deleted tone control position but the splines on the knob doesn't match my stock knobs so I'm sticking with the toggle for now. A peice of copper wire wrapped around the base of the switch makes a good bushing to fill the hole.
After rounding up the switch, I addressed the issue with my electrocution screw. :icon_biggrin: I'm not a big fan of plastic or nylon screws and don't have any on hand, so I took a slightly different approach that hopefully meets the same objective. I am awash in standoffs of all shapes and sizes. Found one that uses 2 mounts that straddle the chassis mounting screw. drilled 2 new holes and bolted it to the board, so now the standoff is truly isolated with no conductive parts hanging out the back of the chassis. That's about as far as I got last night.
We have some guests coming for the weekend so my wife is demanding I clean up the house and put things in order :l2: This will limit my time on amp projects but hope to get back on this Sunday afternoon and finish up the mods as you have drawn. Looking forward to hearing how it sounds.
Thanks for taking so much time and interest in this project and mentoring me through this. In case it isn't obvious, I'm having a ball with this and most of all learning a great deal.
This Forum rocks!
-
So I finished up the mods Sluckey concocted for the Airline. There are a few things I would like to clean up and tweek but overall I am happy with the result.
I relocated the standoff mounting screws. Installed the 2.2uf caps, deleted the tone controls, relocated the 1M, installed sheilded cable from input jack to board, installed "hot"/"normal" switch.
We tested it out last night. Using the 15" speaker that was in it, it was still dark and bass heavy. Tones ranged from very clean to extreme breakup. My son says it gets "farty" when you max out the gain.
Next I hooked it up to a pair of 12" organ speakers. On these it was extremely bright, almost harsh and peircing. Of course these were not in a proper cab, just mounted on a baffle board so some bass cancellation was to be expected.
Then I hooked it up to the cab we built for the Webster/Allen amp. using 1-15" and 1-12" Here is where it really came out with some nice balanced tone and breakup.
Last I used the 15" built in and added just 1 of the Baldwin 12" It is livable but not as exceptional as I think it could be with the right speaker. I suspect in these experiments I was playing a bit fast and loose with impedance match. There are 2 taps on the transformer, not sure which one is 8 ohm and what the other tap is. No markings or indication. I am going on assumption they stayed on the 8ohm when they changed the original speaker out.
Need to figure this out before I go speaker shopping.
I'm thinking I may try 1uf caps in place of the 2.2uf, see if it lessens the "fartiness"
Also going to change out the current 12AX7's to some of my Baldwin/Raytheon black plate 12AX7A's as well.
This has become a great test/experiment project.
-
Maybe keep the cap on pin 3 and remove the cap from pin 8? You don't need to insulate the lead on those 2.2K resistors since those leads connect to ground.
-
Spent a little time on the Airline last night trying to finish it up.
First job was to replace the pressboard baffle. It now has a plywood baffle board.
I replaced the 15" speaker with a 12"speaker so I can get a back on the cab now and it will hopefully roll off some of the bottom end and add a little more sparkle which was lacking with the 15.
Put the screws back in the lower chassis and secured it to the cabinet again.
Last job was to replace the standard 1/4" jack with a 12A style jack and fill the empty jack holes.
Once that was all done, I buttoned it all up with great expectations....
Of course nothing goes as planned. I screwed the pooch somewhere... I am now getting a severe 60hz hum as soon as any volume pot is turned up past 2. The hum becomes louder if I touch the 12AX7 preamp tube glass, or the screws in the isolation mount for it. Guessing I hosed up the grounding somewhere along the line. It does have signal and it does amplify but it doesn't sound good with all the hum. A couple times it started to squall a little for no apparent reason. Sounded like feedback but no signal to cause feedback and it was self-sustaining. My son said it sounded like I was trying to communicate with whales. LOL. I also noticed I have a faint blue glow coming from one of the 6V6 tubes. Just one of them. :w2: It will have to come back apart again. :BangHead:
-
Found the problem. Made a mistake when I switched out the jack. It is back to gether and working nicely now. The 12" speaker sounds a lot better than the 15 did. Considerably brighter and more balanced. Not thrilled with the volume controls and tone control. Might be a future project to work on these. Thinking it might work to put a tone stack in using channels a and b tone controls as bass and treble rather than one as a cut and the other just diasabled... I have plenty of gain to play with now. It could actually use a little taming. Will try to get a sound clip/video done and posted. Besides the control issues I am pretty satisfied with it. It needs a Tolex job. I have never ventured into that territory so that will be a first.