Old tube amp modding

**SamanthaCay** · #1 03-19-2010, 04:11 PM

Ok so I have this old Kay model 720 bass amp dated 1963. I’ve had it for almost ten years. As of the last couple of years it’s had an intermittent hum that comes and go’s. So today I decided to see if I could get to the bottom of it. Now I knew this thing had at least had the power transformer replaced at some point as well as the power tubes for reasons that where obvious. So come to find out there where a few splices here and there that had been covered with electrical tape among some other things. the wires that connect the fuse power switch and pilot light where a little to short as they would just dangle inside the amp. Running from the chassis at the bottom of the cabinet up to the face plate at the top. So in order to secure the wires into the corner of the cabinet I decided I would start by changing those out for some longer ones. When I peeled the tape back on one of the connections I found the two wires where just simply twisted together with no solder! So here’s hopeing that was what caused the hum problem. I then ran through it and checked for more shouty wiring and or cold solder joints and found none. As of now the thing is looking pretty sound and I am looking for some advice as to how I can mod this thing to get more highs out of it. that is to say that the amp sounds just a little to dark for my taste. I was wondering if there was maybe a cap resistor or pot etc.... that I could replace with a different value for a change in tone. So anyways I have some pic’s I took of the guts and of the schematic that is stuck inside the cabinet. It’s only partial because of damage and there is a parts list in full next to it that I can’t get a good pic of for the life of me. Although I can certainly read it. So if you have a question about a certain component just ask me and I can look and fill you in on what it is. Also I noted that the inputs are linked via the hot by some resisters. I don’t know but would it be possible to change those out for a lessor value or just due away with them altogether thus achieving some more top end?

Samantha.

chokeslam512 · #2 03-19-2010, 04:46 PM

No help from me but the schematic reminds me of the map from "The Goonies"

KramerBassFan · #3 03-19-2010, 04:59 PM

Yes you COULD try swapping out a few different resistors on the inputs....

And for everybody else onlooking... that is REAL POINT TO POINT.. not necessarily so nice, now is it?

Not too bad on this small amp... but imagine the real birds nest it would be if it was a larger amp!

BillyB_from_LZ · #4 03-19-2010, 05:16 PM

Hum might be caused by old dried up filter capacitors in the power supply (they're probably metal cans with two caps in them...looks like they're called "C3" in the schematic (in a couple places)). Could also be caused by poor bonding between the jacks and the chassis...try removing them and cleaning up the area with steel wool.

Boost the highs? Perhaps cap "C1" between the preamp tube grid and ground is shunting highs to ground.

Try PMing Bassmanpaul...he's quite good with this stuff.

jimbilly · #5 03-19-2010, 11:24 PM

Like stated above, it likely needs some capacitor replacement, a good tech will check it out and not charge a huge amount for a cap job and service. I'll bet if you ran it into a more modern bass cabinet it would sound a lot more modern, without any mods to the circuit. I was just playing a 2x6L6 amp into a modern 4x10 bass cabinet, 50 watts is pretty loud and big through a good 4x10.

T-Bird · #6 03-20-2010, 04:19 AM

Hi.

A really cool little amp.

For some odd reason, www.schematicheaven.com doesn't seem to have that particular schematic anymore. Since You have the original, it doesn't matter that much though.

A true PTP is a PITA to repair, even as simple circuit like on Your Kay, but with patience, any mods are possible.

In a case like this, I usually start by modding the tone circuit(s), and if that doesn't yield desired results, then dig in deeper.

Be safe, don't get zapped, etc.

Regards
Sam

**SamanthaCay** · #7 03-20-2010, 04:49 AM

Wow you guy’s are really coming trough on this one and in only fore posts! Thank you so much for the sound advice. As far as the concern over the filter cap this is what my amp tech told me, he’s the guy I go to when all else fails or if I’m feeling lazy and I have the money. I wanna say it’s been about a year or so since I took it to him for an opinion on what that weird intermittent hum might have been. When I got it in his shop the hum did not want to come out, so he took a brief look at it and said that it was most likely a loose connection somewhere. I asked about the filter cap and he said it was not likely as in most cases when one dries up it will create a constant hum that progressively gets louder in time before finally giving out. So based on that and the fact that at some point in this amps life and more then likely on a few occasions it’s had a fair share of replaced parts. Consisting of as far as I can tell the power and output transformers, power tubes, fuse holder, the shoddily placed wires that I mentioned in the op and the thing is dated 1963. So although I’m not 100% sure that cap’s been replaced at some point, I’m willing to bet that it’s not going bad anytime soon.
Alright now for the good stuff.
I took the advice of both changing c1 off the pre amp grid and as well I changed out two of the resistors on the inputs. I went from a .1 to a .047 for c1 and with the resistors I did a 250k, 470k and left the third input stock at 10,000k. As far as I can tell by plugging into input 3 the new cap isn’t making any noticeable difference. The new resistors on the other hand made a huge difference. input one sounds quite a bit louder and has a whole lot more treble yea! The hum is gone as far as I can tell as I played through the thing for about two hours tonight with no issues. I’m am so happy with this thing now I might even try it out in a band setting. The only thing I’m gas’ing for now is a place to crank it up, I live in an apartment. I did get it about 3/4's of the way up today before my neighbor knocked on my door and to my surprise the thing sounded pretty clean where as before not so much. so I guess that’s all for now, I’ll try and post some pic’s of the whole amp in the next couple of days and maybe some sound clips.
Thank you guy’s I couldn’t have done it with out you, this is why I love talkbass

Samantha.

MAMMOTHvolume · #8 03-20-2010, 07:15 AM

looks like that amps already had quite alot of work done on it in the past! looks like power caps have already been changed. Have you checked the power resistors to make sure they havn't drifted to much? The non ceramic cap on the pot in the second picture looks like it could do with changing aswell

KramerBassFan · #9 03-20-2010, 07:52 AM

Glad to see replacing the input resistors helped you get what you wanted.

**SamanthaCay** · #10 03-20-2010, 08:15 AM

Quote:

Originally Posted by MAMMOTHvolume

looks like that amps already had quite alot of work done on it in the past! looks like power caps have already been changed. Have you checked the power resistors to make sure they havn't drifted to much? The non ceramic cap on the pot in the second picture looks like it could do with changing aswell

I’m not sure exactly what you mean by the power resistors, as I’m pretty novice at this stuff. I’m guessing you might be referring to the to larger ones on the 5879 preamp tube and on the filter cap? I did not check those and now that it’s been mentioned you got me wondering. Do you think I should, even though everything seems to be work.ing fine as of now? And as far as the cap you are referring to on the tone pot did you mean the ceramic one or the old oil paper one next to it? From the look of the schematic it would seem that the ceramic one is non existent. As in someone probably put it there to make up for the old oil paper one that had dried out and just negleted to remove the old one. In any case it works and sounds pretty good so I’m pretty happy for the time being. I was on the other hand also thinking about putting in a resistor or something to run the speaker at 2 ohms as it is an 8 ohm driver and I would like to push as much power as I can out of this thing. Or maybe I’ll just shell out the cash for a better driver. Last I thought I could also maybe ad a tweeter and crossover in there to possibly extend the dynamic range and get more power off the amp. What do you guys think? Any suggestions are certainly appreciated.

Samantha.

KramerBassFan · #11 03-20-2010, 09:00 AM

I'd say just run that amp into a bigger speaker cabinet.

Don't tinker with the speaker, and especially not the output transformers tap.

**SamanthaCay** · #12 03-21-2010, 05:04 AM

Here’s a youtube video I made today. It’s probably not the best example as to the natural sound of the amp as I was using my wah and delay and the wah really pushes the thing into overdrive. Along with the crappy camera mic it doesn’t sound quite the way it does in person but is a cool video none the less.

BillyB_from_LZ · #13 03-21-2010, 07:37 AM

Quote:

Originally Posted by buchananbass

Here’s a youtube video I made today. ...

Link? (I searched for Kay Model 720 and didn't find anything.)

**SamanthaCay** · #14 03-21-2010, 07:44 AM

Quote:

Originally Posted by BillyB_from_LZ

Link? (I searched for Kay Model 720 and didn't find anything.)

Ok that was dumb of me, here ya go.
http://www.youtube.com/watch?v=al3Q7DWl0n0

**SamanthaCay** · #15 03-21-2010, 07:45 AM

Pic’s! I’ve still got a few loose ends to tie up but I figured I’d post some pic’s of it as it sits. Look at it it’s just like new.

BillyB_from_LZ · #16 03-21-2010, 07:56 AM

Thanks! Interesting videos!!!

KramerBassFan · #17 03-21-2010, 10:08 AM

Really cool amp from an era long gone now.

**SamanthaCay** · #18 03-21-2010, 03:27 PM

Hey if you guys ever see one of these things for sale let me know. I’m kinda thinking it would be nice to have six of them or maybe just two. Also I figure if I could find one on the bay or something at least I could get a better idea of what their worth. It’s not like I search for it all the time but in the ten years I’ve had it I have only seen one on ebay and it was in worse shape then mine. I can’t seem to find much info on it in general really.