Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Duncan on December 16, 2022, 05:39:21 pm
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Howdy gang.
I recently completed my first amp build (a TW Express circuit). Everything fires up, and I'm getting signal through my speakers, though it's very VERY quiet (as in, with the volume on 10, I'm only as loud as someone talking loudly). It's also very muffled. I currently have a full compliment of EL34's and 12AX7's known to be good loaded in there.
I've gone through the amp to make sure no connections are shoddy or missing, and I can't find anything.
and before anyone offers insight about lead dress or component placement, I know. Ken Fisher did it better. My goal is to have an amp that fires up and makes the right amount of volumes. I’ll worry about stability and noise once the volume issue is sorted.
Here’s an album of photos that also includes my voltages:
https://imgur.com/a/IWKdUX8
Note: the heater voltages were measured at each pin, so I'm really getting 5.6vac to the heaters.
I’ve done a pop test (checking voltage at the plates to see when the sound from touching the probe drops) and V1 is noticeably quieter than all others.
I’ve checked the impedance on both sides of my OT primary, and it’s good.
I've also confirmed my input jack is wired correctly.
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You may want to post a link to the lengthy thread over on TAG. Lotta history on this amp.
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Hi there
I have a thread going on TAG as well, but I’m finding there’s only a couple people willing to help (I think because I didn’t try to make a 1:1 Francesca replica, or because my questions are too basic because I’m new to this).
I figured I’d hit up another forum that has a whole bunch of really knowledgeable people
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Not that I have a solution, but I just wanted to say that your first build looks really good. There are people that have built alot of amps that do far worse at lead dress etc.
One possible culprit of trouble could be the bus wire on the back of the pots. Soldering doesn't look solid and one could probably wiggle it loose. Alpha pots require a little prepping for the solder to stick to them. Some abrasive/sanding paper and a good iron is needed for that. The casings on Bourns and CTS pots are way easier to solder on.
/Max
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Not that I have a solution, but I just wanted to say that your first build looks really good. There are people that have built alot of amps that do far worse at lead dress etc.
One possible culprit of trouble could be the bus wire on the back of the pots. Soldering doesn't look solid and one could probably wiggle it loose. Alpha pots require a little prepping for the solder to stick to them. Some abrasive/sanding paper and a good iron is needed for that. The casings on Bourns and CTS pots are way easier to solder on.
/Max
I did sand the backs before putting the wire on, but I’ll definitely take a look and see about touching it up.
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Checked the bud wire. Solid contact across the board.
I’m honestly at a loss
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I have a thread going on TAG as well, but I’m finding there’s only a couple people willing to help (I think because I didn’t try to make a 1:1 Francesca replica, or because my questions are too basic because I’m new to this). I figured I’d hit up another forum that has a whole bunch of really knowledgeable people
There are a lot of knowledgable folk here, but to be fair, you've had the full engagement of Martin Manning and Sluckey over at TAG.
A schematic is always helpful, and is actually the language of choice for this forum.
If this were my amp, based on the TAG thread, I'd take it apart, verify the schematic, then create or verify a new layout, and only then rebuild. Not sayin' this is the only way, but it is what I would do. There will be other opinions, of course.
It's a very complex amp for your first build. Pedals are entirely different animals.
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I have a thread going on TAG as well, but I’m finding there’s only a couple people willing to help (I think because I didn’t try to make a 1:1 Francesca replica, or because my questions are too basic because I’m new to this). I figured I’d hit up another forum that has a whole bunch of really knowledgeable people
There are a lot of knowledgable folk here, but to be fair, you've had the full engagement of Martin Manning and Sluckey over at TAG.
A schematic is always helpful, and is actually the language of choice for this forum.
If this were my amp, based on the TAG thread, I'd take it apart, verify the schematic, then create or verify a new layout, and only then rebuild. Not sayin' this is the only way, but it is what I would do. There will be other opinions, of course.
It's a very complex amp for your first build. Pedals are entirely different animals.
It might come to a rebuild, but I want to see my current iteration through before ripping it apart.
or those just joining up here’s a recap:
Amp passes signal, but very quiet and muffled
voltages are good throughout
Ground contacts go to ground
Several tubes known to be good have been tried (both preamp and power tubes, biased to 60-70% plate dissipation using Bias King - will install 1 Ohm resistors to confirm bias levels through a manual process this week)
Components and chain have been checked and double checked
Shielded cable from input has been swapped for regular wire
Pop test on plates nets a much quieter result at V1
All connections have been cleaned up and reflowed
Removing bright switch removes all signal
Bypassing tone stack removes signal
All preamp sockets have been rewired and double checked
Different speaker and speaker cable combinations make no difference
Input jack has been rewired and confirmed to have 1M resistance from tip to ground
Output transformer has been checked (resistance between centre tap and each side is 43 ohms)
The two bolded items consfuse me, but maybe they help narrow down where things are getting dumped?
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Removing bright switch removes all signal
Bypassing tone stack removes signal
If removing bright switch removes all signal then you must have made a mistake when removing it or you have a simple wiring error.
Exactly how did you bypass the tone stack? Be specific.
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It would be helpful if you could identify the schematic you are working from.
Is it from one of these? https://el34world.com/charts/Schematics/files/Train_wreck/Trainwreck_express.pdf (https://el34world.com/charts/Schematics/files/Train_wreck/Trainwreck_express.pdf)
Based on your bolded symptoms, I'm going to guess that there is a wiring error in the vicinity of your volume pot and bright switch, or maybe somewhere within the tonestack. I can't tell from your photos.
Look at your components. Where does each component lead go to? Does that agree with your schematic? If yes, then move on to the next component. If not, well then . . . Saying this is a tedious process does not do justice.
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It would be helpful if you could identify the schematic you are working from.
Is it from one of these? https://el34world.com/charts/Schematics/files/Train_wreck/Trainwreck_express.pdf (https://el34world.com/charts/Schematics/files/Train_wreck/Trainwreck_express.pdf)
Based on your bolded symptoms, I'm going to guess that there is a wiring error in the vicinity of your volume pot and bright switch, or maybe somewhere within the tonestack. I can't tell from your photos.
Look at your components. Where does each component lead go to? Does that agree with your schematic? If yes, then move on to the next component. If not, well then . . . Saying this is a tedious process does not do justice.
I compared the schematics you linked, and the one I'm using (from the Build Guide penned by Ron Worley) is a different one, slightly. I've attached it to this post.
From what I can tell, there are many differences with the schematic I'm using and the ones you posted including ot values, the bright switch isn't in the first several versions above, the 820R grid stopper on V1 (the one I'm using doesn't have it), the second bypass cap on mine is a 22uf and on A0 is 220uf, and a few more component values in the phase inverter/power supply section.
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Have you tried doing a pop* test on each of the grids, starting with the output tube grids? You need to eliminate which stage might be causing the problem.
*contacting the grid connection with (an insulated) probe
"Weak signal' could mean a number of things, e.g.:
- mis-wiring
- wrong value resistor(s) somewhere (like a bias resistor, grid leak resistor or plate resistor) - check each resistor value with multi meter
- leaky coupling cap on one of the stages (as in 'DC leakage' across the cap, which is not the same thing a leaky solid/liquid),
- partially fried OT
- something else etc
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Have you tried doing a pop* test on each of the grids, starting with the output tube grids? You need to eliminate which stage might be causing the problem.
*contacting the grid connection with (an insulated) probe
"Weak signal' could mean a number of things, e.g.:
- mis-wiring
- wrong value resistor(s) somewhere (like a bias resistor, grid leak resistor or plate resistor) - check each resistor value with multi meter
- leaky coupling cap on one of the stages (as in 'DC leakage' across the cap, which is not the same thing a leaky solid/liquid),
- partially fried OT
- something else etc
Hi there
Yeah I’ve done a pop test on the plates and all of them make an appropriate pop except for v1. I have a low-key suspicion it might be a leaky bypass cap. I’ve confirmed all of the resistor values (via colour bands) but I reckon it’s time to go through them with my meter one by one.
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Yeah I’ve done a pop test on the plates and all of them make an appropriate pop except for v1. I have a low-key suspicion it might be a leaky bypass coupling cap.
See procedure for testing DC-leakage in coupling caps @ Fig. 4-4 on p57 of the attachment. Note that the meter reading will show DC as the cap charges up when you first switch the amp on (which is normal because the voltage on the other side of the cap is changing during start-up), but when the DC voltage on the plate-side lead of the coupling cap has stabilised at its normal high VDC quiescent (idle) voltage, a healthy coupling cap should show 0 VDC on the opposite lead. Any hint of VDC, and you should replace the cap. DC from a leaky coupling cap affects the bias-point on the following tube stage (and this reduces gain).
(Note, this procedure involves high-voltage testing of the amp, so take all appropriate precautions in order to avoid electrocution by using insulated gator clips/test-clips and not touching anything connected to the inside of the amp when it is switched on etc . You have been warned)
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Best of luck!
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ITS FIXED.
I had wired the B+4 lead to the wrong side of a resistor.
Holy hell I feel dumb. I greatly appreciate everyone’s input and help.
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Great! You should be proud and happy. I can see it in the pics now that you pointed it out. :wink: