YBA-2B Cab rehab - removing a glued brace?

[cdlynch]

I'm trying to barely salvage a water damaged YBA-2B cab, and making some changes while I'm at it.

For internal bracing it has a couple of ply strips glued to the back top and bottom. I tore out the rotted bottom strip without trouble. The top one is still strong, but I want to enhance the bracing with top and bottom one-piece rectangular braces cut from my scrap ply. (image below) I'll add vertical corner strips as well for overkill. I'm replacing the 15" with a 12" because the cab volume is perfect for the Eminence Basslite S2012, even with extra bracing.

To add the top rectangle I'll have to remove the good brace, but the glue joint is strong. It's ply glued to fiberboard top and back, so if I just brute force pry it, I assume the fiberboard will fall apart.

Wood glue is usually stronger than wood, so I'm hoping maybe heat will help me pry it out cleanly.

Anyone have experience with this? Am I looking at hours with chisels instead of heated blades?

 

[popgadget]

Link Removed

 

[cdlynch]

Cool, never considered one o' them before. I just got a great Makita jig saw so my tool budget took a hit, but that's probably the best way forward. Thanks!

 

[popgadget]

I call it the eraser. It makes mistakes disappear!
Also very handy with a sanding "finger" for getting those hard to reach areas.
There is a corded version as well, but I've got a lot (most) of their 18v cordless tools, so the cordless was a no brainer.

 

[cdlynch]

I couldn't afford "The Eraser" so I carefully chiseled out that remaining brace and cut new ones. The new baffle board is cut for the 12" Basslite, and I'm as far as the dry fit stage. I'm going to glue and screw the whole thing together. The only visible difference externally will be the screws and countersunk finishing washers. I have to find the best places for screw holes, and then figure out the best order for screwing it in once glued, which'll be a bit of a puzzle. I'm using Titebond III for the long setup time.

The amp needs a new power supply, and maybe some other tweaks after that, but is salvageable and should sound good once tuned. Considering the initial condition (see pics), I payed too much for the amount of work I'm doing, but I'm learning plenty and will have a nice recording amp in the end.

 

[cdlynch]

I think it's going to live!



On the amp side, the power transformer needs to be replaced. It's running way high voltage in part because wall voltage is higher these days, and because there's just something wrong with the old transformer. It's worth noting that the voltages called for in the schematic



are too high for standard EL-84/6BQ5 tubes, but Sovtek military spec tubes are perfect, and the only ones I know of that will handle the high voltage.

 

[FlaCharlie]

I think it's going to live!

View attachment 804727

On the amp side, the power transformer needs to be replaced. It's running way high voltage in part because wall voltage is higher these days, and because there's just something wrong with the old transformer. It's worth noting that the voltages called for in the schematic

View attachment 804728

are too high for standard EL-84/6BQ5 tubes, but Sovtek military spec tubes are perfect, and the only ones I know of that will handle the high voltage.

What, besides the high voltages, makes you think there is something wrong with the power transformer? If the transformer is OK, there are other, cheaper, ways to drop the voltage.

I own one of these, an earlier version that I found on Craigslist several years ago. I rebuilt one of the guitar versions for a friend a few years ago but I haven't opened up the amp on mine yet, although I do use it occasionally. Most of my experience is with tube stereo but it was common to run tubes really hard.

The Sovteks are tough, but the prices at the link you included are pretty high. These guys Vacuum Tubes prices. Best prices every day! list them for $11. They're right down the road from me. I prefer American tubes in stereo applications, not sure how much difference they would make in a bass amp.

I'm interested to see how your speaker swap goes.

 

[cdlynch]

What, besides the high voltages, makes you think there is something wrong with the power transformer?

My amp tech friend said the same thing and did various tests before determining that it was out of spec. First he used a variac to deliver what the circuit expected, and when it was still high, he tried other things to reduce the voltage and finally said it's actually bad and can't be worked around. He's not charging for this, so there's no incentive to lie.

The $11 tube doesn't look to be the EL84M version unless I missed something, just the standard. Tube Depot has the non-military tube for about $10.

Since my head won't be ready for a while, the only testing I can do for now is with a heavily modified Valve Jr. head, which has a bass-capable OT. It'll just have garbage headroom.

Once I've plugged every possible leak I'll stuff it with poly fill and call it done.

EDIT: Successful test of the cab with the Valve Jr. It's stoopid loud and much cleaner than I expected. I should have known, as that head is usually paired with a terrible 10" cone from a 80's "hi-fi" which probably has horrible efficiency. I might have to pick up a Basslite S2010 for that cab.

 

[lelandcrooks]

Harbor Freight has a similar tool for about 30 bucks. It works very well. Corded.
What you can do with well sharpened chisels though is amazing.

 

[FlaCharlie]

My amp tech friend said the same thing and did various tests before determining that it was out of spec. First he used a variac to deliver what the circuit expected, and when it was still high, he tried other things to reduce the voltage and finally said it's actually bad and can't be worked around. He's not charging for this, so there's no incentive to lie.

The $11 tube doesn't look to be the EL84M version unless I missed something, just the standard. Tube Depot has the non-military tube for about $10.

Since my head won't be ready for a while, the only testing I can do for now is with a heavily modified Valve Jr. head, which has a bass-capable OT. It'll just have garbage headroom.

Once I've plugged every possible leak I'll stuff it with poly fill and call it done.

EDIT: Successful test of the cab with the Valve Jr. It's stoopid loud and much cleaner than I expected. I should have known, as that head is usually paired with a terrible 10" cone from a 80's "hi-fi" which probably has horrible efficiency. I might have to pick up a Basslite S2010 for that cab.

I didn't realize you weren't working on the amp yourself. Since that is the case, my questions are probably moot but I'm curious.

Do you know how high the voltages were? Or what "other things" he tried?

I'm not an expert on transformers but I can't figure out what would cause a transformer to go "bad" and put out more voltage. Less or none, sure, but more?

Most vintage gear, when new, was considered to be "in spec" if the voltages were +/- 15% of the voltages on the schematic. If he tested the transformer by itself, separate from the circuit, it will be much higher because there is no load on it. If it was more than 15% higher when connected to the circuit and with the variac set at 115v, it's much more likely to be out of spec parts elsewhere (typically a lot of them) which are resulting in reduced current being drawn and, therefore, higher voltages. First place to look is the bias circuit on the 6BQ5s.

If the other parts are in spec - if they are original, they won't be so I should say . . . Once you replace the out of spec parts, if you want to reduce voltages further to make up for the higher wall voltages that are common today, a minor modification of the power supply - adding a cap and resistor right after the diodes - should do the trick.

So, how is the frequency response with that speaker in the cabinet? I tried a couple of random 15" speakers as replacements in mine but they were obviously not suited to that, fairly small, sealed cabinet so I went back to the stock speaker.

 

[cdlynch]

I didn't realize you weren't working on the amp yourself. Since that is the case, my questions are probably moot but I'm curious.

Do you know how high the voltages were? Or what "other things" he tried?

I'm not an expert on transformers but I can't figure out what would cause a transformer to go "bad" and put out more voltage. Less or none, sure, but more?

Most vintage gear, when new, was considered to be "in spec" if the voltages were +/- 15% of the voltages on the schematic. If he tested the transformer by itself, separate from the circuit, it will be much higher because there is no load on it. If it was more than 15% higher when connected to the circuit and with the variac set at 115v, it's much more likely to be out of spec parts elsewhere (typically a lot of them) which are resulting in reduced current being drawn and, therefore, higher voltages. First place to look is the bias circuit on the 6BQ5s.

If the other parts are in spec - if they are original, they won't be so I should say . . . Once you replace the out of spec parts, if you want to reduce voltages further to make up for the higher wall voltages that are common today, a minor modification of the power supply - adding a cap and resistor right after the diodes - should do the trick.

So, how is the frequency response with that speaker in the cabinet? I tried a couple of random 15" speakers as replacements in mine but they were obviously not suited to that, fairly small, sealed cabinet so I went back to the stock speaker.

I don't remember exactly, but I think what we saw was in the neighborhood or higher than this schematic from elsewhere (which suggests that the problem is not unique to my amp), and it was tested in the circuit. I was there and paying attention, but I know very little, I'm just an interested bystander trying to learn bits as I go:

He tinkered with the bias circuit and caps/resistors in the power supply, mostly. He didn't see evidence of other components being out, and most of the amp was recapped before I bought it.

As for speakers, that cab volume is too small for a 15", but back then people just sort of built what they thought would be convenient, from what I've heard. The Basslite S2012 calls for almost the exact volume of that cab, so it should perform well, in theory. In practice it sounded good, but on the dark side until I switched in the bright cap on the Valve Jr. mod. It's so friggin' loud with the 5w amp that I think at 25w, I might be able to gig with the damn thing. Also, there's so much room in the chassis that with a different OT and PT, I could add 2 more EL84s (and whatever circuit mods are called for ) and have a 50w amp. That's a project for another life, probably.

So tube-wise, the schematic calls for 395v for the power tubes. Modern EL84s are rated for 300v, and the Sovtek EL84M is rated at 400v. People run the regular ones in these amps all the time, they just don't last as long or have as much headroom. I figure for a bass application it can't hurt to have the extra guts.

 

[FlaCharlie]

I don't remember exactly, but I think what we saw was in the neighborhood or higher than this schematic from elsewhere (which suggests that the problem is not unique to my amp), and it was tested in the circuit. I was there and paying attention, but I know very little, I'm just an interested bystander trying to learn bits as I go:

He tinkered with the bias circuit and caps/resistors in the power supply, mostly. He didn't see evidence of other components being out, and most of the amp was recapped before I bought it.

As for speakers, that cab volume is too small for a 15", but back then people just sort of built what they thought would be convenient, from what I've heard. The Basslite S2012 calls for almost the exact volume of that cab, so it should perform well, in theory. In practice it sounded good, but on the dark side until I switched in the bright cap on the Valve Jr. mod. It's so friggin' loud with the 5w amp that I think at 25w, I might be able to gig with the damn thing. Also, there's so much room in the chassis that with a different OT and PT, I could add 2 more EL84s (and whatever circuit mods are called for ) and have a 50w amp. That's a project for another life, probably.

So tube-wise, the schematic calls for 395v for the power tubes. Modern EL84s are rated for 300v, and the Sovtek EL84M is rated at 400v. People run the regular ones in these amps all the time, they just don't last as long or have as much headroom. I figure for a bass application it can't hurt to have the extra guts.

Just looking at it quickly, the voltages shown are technically within spec (a B+ of 438v is +9.5%) and pretty much what I would expect given the higher wall voltages that are common today. The bias which, I think was -15.5v on the other schematic, is out of spec. Adding the 2.2k screen resistors is a good idea. The 10 ohm resistors allow you to check the bias easily. Adding an accessible test point and a pot to make adjustments would be best.

As far as "evidence of other components being out, and most of the amp was recapped before I bought it" . . . hopefully "evidence" means he measured everything, not just that there wasn't anything that was obviously burnt up. Components that look fine are frequently way out of spec after sitting in a hot chassis for years. I would also definitely replace the coupling caps (C9 and C10) with ones that are rated for 630v - typically the next highest rating over 400 - even if they do measure close to the 0.1 uF spec.

Dropping the B+ voltage a bit would be a good goal but there's no need to replace the transformer to do it. Just add a resistor, or a cap and resistor, between the diodes and the first PS cap (C14) and the voltages going to the 12AX7s will drop too. I'd be comfortable dropping the B+ from 438v to around 360v or so. Another, more involved, solution would be to convert the amp to tube rectification using a rectifier tube that will drop the voltage about 50v or more.

As I said before, I don't know if using American tubes would make as much sonic difference in a bass amp as it does in a stereo amp. The Russian military version is a good call but even regular American 6BQ5s are pretty tough. I'm not sure I would trust regular, current production, versions. If you kept the original PT, I'd also make sure the heater voltage isn't too high. Again, adding resistors is an easy fix, try to keep them at or a bit below 6.3v. There are also alternative tubes that will drop right in, work well and are cheaper. PM me if you want more info on that.

Oh, I'm pretty sure the BassMate is listed as a 20w amp although that might even be optimistic. If you want more than that the best bet would be to build something from scratch, probably using 6L6 family tubes. Another thing to remember is that twice the power will only get you 3db more - using the same speaker setup. So switching to a speaker that is 3db more efficient has the same effect as twice as many watts.

Keep us posted as to how the Basslites perform. Are they designed for a sealed cabinet?

 

[cdlynch]

I asked my friend for more info, but yes, he measured everything, and tested different resistors and caps in several places. I'll let you know if he remembers and gives me more details.

Keep us posted as to how the Basslites perform. Are they designed for a sealed cabinet?

The Eminence specs for the Basslites says they're acceptable for use in sealed or vented cabs. By the numbers they're probably better for vented, but this cab is too small for venting.

 

[cdlynch]

@FlaCharlie, thanks for all your suggestions! So a little update: My friend reports that the voltages, though he doesn't remember exact numbers, were significantly higher than the ones listed above on the modified schematic, and everything was under load while testing. He agrees with and shares your troubleshooting and testing approach.

 

[cdlynch]

@FlaCharlie - I used it for a mini-gig with the 5w head, and it kept up great with guitar, vox, harmonica, and kick/snare/tambourine. It's a woolly, classic sound (not shredded), and I can't wait to get the actual Traynor head back on there. I know it won't gain much in volume, but it'll have better headroom. I've seen the YBA-2A/B listed in several places at 25w, but I assume it's probably closer to 18w, but it'll be interesting to measure. To double volume, you need 10x the watts. My normal gigging unit is a V4-B which, as set up puts out about 89w and can crush any (rationally sized) rock venue in Boston or NYC. So at 18w I could probably do pretty well as long as I want more dirt. More options are always good, but if nothing else it'll be nice for recording and small gigs.

On a Log plot (shows the loudness relationship of watts), the difference between 18 and 25 is pretty small.