Anyone had a similar valve amp issue?

[GregL]

Hi Stu,

It so happens I have a few pics of it at work!

IMG_2975b.jpg (44.09 KiB) Viewed 19000 times

IMG_2976b.jpg (43.45 KiB) Viewed 19000 times

IMG_2980b.jpg (46.47 KiB) Viewed 19000 times

Thanks,
GregL.

[needsmorecowbel]

That's really Nice Greg!! I'm thinking dovetails are the way to go visually (if I go down this head building path). Are these hand cut? or machined via table saw/ bandsaw/ router?

I decided I'll get a tech to have a look at the amp and see if he can diagnose the issue...As I've sort of used up all of the fairly basic "non lethal" testing techniques. Maybe I should just plug in the amp and perform a seance (or even hire a psychic) to make a connection with the amp. As a guitarist mate said recently regarding this amp issue:

"Can't help you, I'm afraid. Analogue circuits run on moonbeams and magic as far as I understand them."

[GregL]

Hi Stu,

Hand cut dovetails.
I'm afraid I'm old fashioned, I don't have any digital devices in my electric guitar signal chain - guitar -> valve amp.

[needsmorecowbel]

Very cool. There's nothing wrong with that at all...

Stu

[needsmorecowbel]

Might have to be a tolex affair on baltic birch ply, Greg, as the hard wood I would liked to have used is no where near dry enough...

[Crafty Fox]

Wow, that amp is classy, or should that be Class A? Very nice indeed.

[needsmorecowbel]

Greg, I remembered a neighbour (who used to repair the electronics on F 111s in the Air Force) is very good with electronics. He said he could have a look at it tomorrow and show me how to bias the new tubes in future. He also saw the design flaw with having the combo amp as a combo rather than separate head and cab. His suggestion was if we fix the problem to remove the valve head and make a new box for it. Huzzzah! Progress!

[GregL]

Ken,
I think the 5E3 (Fender Deluxe) that this amp is a clone of, operates in class AB, though another valve amp I made is class A.

Stu,
Biasing output valves is a useful thing.
I think the F111 was first made in the late '60's, so your neighbour might be familiar with valves, although valve guitar amps are not like the complex electronic devices likely to be seen in F111s!
You can use any timber to make head and speaker cabinets, the dimensions are so small you don't have to worry about material strength.

Thanks,
GregL.

[needsmorecowbel]

Cab internal size will be 635mm long, 240 mm wide, 285mm tall. + the thickness of the stock...Taller than your average head as it was designed to stay in the cab...

[needsmorecowbel]

Something Like that is what I'm thinkin...or just a similar maroon tolex. Whatever is easiest to get...

[Nick]

If you aren't averse to a little shopping "off shore" Stu, here's a link for most things valve amp. I've linked to the Tolex page first though.
http://www.vibroworld.com/parts/tech11.html

[needsmorecowbel]

Cheers Nick!

[ozziebluesman]

G'day Stu,
I have a similar hand built amp to GregL. It was made by Duane Ledford in Sydney and is nine watts running 6V6 power tubes and by changing the rectifier it could pump out 22 watts. The circuitry in this amp is first class and the amp is noiseless even when you crank it, however at certain frequencies there is harmonic valve rattle. It is just noticeable but in a more powerful amp it would probably be very noticeable. I think the rattle goes with the territory and something that most valve amps experience. I could be wrong there only speaking in my experiences. It is a beautiful, tone full, responsive little amp and I love it!

Just sharing my experiences.
Cheers
Alan

[needsmorecowbel]

Thanks for the tip Alan, That is a nice setup you have there.

I had an electronics expert look at the amp today:
We bypassed the internal speakers by plugging in an external speaker and the rattle was still audible through the external speakers which were a good 3m away.
Therefore it has to be something within the chassis (tubes/ FX (reverb)/ something in the circuit) causing input distortion/rattle and the distortion/rattle is not directly caused by speaker vibrations...Finally getting somewhere close to diagnosing the issue.

The electronics guru I used couldn't hear the distortion/ rattle as he's lost those frequencies (after long term exposure to jet engines) but he didn't charge me to have a look at it and go through almost an hour of testing. He did suggest changing some of the capacitors as there was very slight hum with the volume knob set to 0 on all channels.
So:
Most likely - Reberb Tank could possibly be cracked, leaking/ broken
Quite Likely -Power Tube possibly beginning to fail
Speakers are not the problem (but of course can reduce the life of the tubes/ reverb tank through vibration).
Could replace capacitors to reduce/remove all hum (but the hum is minimal...and I've never had an amp that didnt have a small level of hum so I'm not fussed)

Next Stop: Amp Technician for testing of the reverb tank and tubes...

[needsmorecowbel]

As you suggest it could be a limitation with valve amps but it really is quite severe in this case...

[needsmorecowbel]

For anyone interested. Just got the amp back from the repairers. He basically outlined what he said last time when I demonstrated the issue on my guitar. He said it was one of the worst cases of mechanical chassis rattle that he had had to deal with but that are all too common amongst combo amplifiers (to differing degrees). He's managed to reduce the rattle quite a bit by packing foam in strategic places and slightly adjusting/ moving the wiring. He agreed that moving the chassis out of the cabinet into a seperate head unit couldn't hurt.

Going to sit on this one for a while

Stu

[needsmorecowbel]

Just a quick update:

I put the valve head from the combo amp into a separate cabinet. This has eliminated the rattling issue entirely up to about 75% volume (60 watt amp). Prior to the mod I couldn't get it past 20% without rattle. It looks a bit silly but it works...Sound file to come in a few days. Still needs glue, sand and polish and I need some advice whether to seal the back of the speaker cabinet to avoid the sound from spilling out the back of the speaker cabinet.

[Nick]

Nice clean job Stu and can be taken back to it's original combo state if you decide to sell further down the line and new owner want's it back as a combo. Mind you, that cabinet should have been constructed of flame Maple, Blackwood bindings and internal spruce bracing at least!

[needsmorecowbel]

Cheers Nick. Yeah too bad I didnt have any brazillian rosewood lying about...

[GregL]

Hi Stu,

Looks fine!

>This has eliminated the rattling issue entirely up to about 75% volume (60 watt amp).

75% of a 60watt valve amp should be loud enough! If you need it louder, get more speakers. Try not placing the head on the speaker box, maybe even less rattling?

>I need some advice whether to seal the back of the speaker cabinet to avoid the sound from spilling out the back of the speaker cabinet.

Depends what music style and what sound you're after. Open back (ie unsealed) is *usually* for less distorted, cleaner styles where the slight "floatiness" of the sound is an asset. Closed back (ie sealed) is perhaps more "constrained" and more controlled by the sound of the amp.

Ever notice how hard it is to verbally describe sounds?

Thanks,
GregL.

[needsmorecowbel]

Cheers for the advice Greg,
I might experiment with closed and open backs and see which sound I like more.
> I didn't make it very clear in my brief post but I only put the head on top of the speaker when taking the photos.
> I've found that when the two are separated by a good metre or two that it is least noticeable but still only just audible when they are stacked on top (It seems that splitting the combo does the trick to make it barely audible). Of course you have to be in a room where there are not loose tools/ items on bookshelves as everything moves around slightly when the amp is cranked up that high.

> I agree 75% of the volume is very loud and I'd rarely have it above 4-5 on the volume knob.

My guitar teacher suggested that I try plug the amp into a 2 x 10 inch speaker cabinet in order to compare the difference in mid, treble and bass response. His opinion was that he was able to get a much "tighter" bottom end response from 10 inch speaker cabs rather than 12 inch speaker cabs. I'd like to give this a crack at some point to see the difference side by side of a 10 and 12 inch cab. Just curious is all. I'm guessing to test this I'd have to ensure the 10 inch speakers had a similar max wattage in order to not damage the cab.

Stu

[GregL]

Hi Stu,

Good idea to compare 2 x 10" with 2 x 12", see what you like best.

>I'm guessing to test this I'd have to ensure the 10 inch speakers had a similar max wattage in order to not damage the cab.

You don't need similar wattage cabs (of course, each cab should be *at least* 60 watt for a 60 watt amp). But you do have to watch the impedance load on the amp. Most valve amps will want to see 4, 8 or 16 ohm impedance. I'm guessing your amp has a selector switch on the back to set expected speaker impedance. Then it depends how you are going to connect the speakers cabs for the comparison - connect both at the same time, or connect one, play, and then connect the other, play (and remember what it sounded like before). It would be best if the impedance of each cabinet is the same. Then if you connect both at once, you need to be aware of how the impedances combine - parallel or series. If you connect one, play, connect the other, play; don't forget to switch amp off between cab swapping - valve amps *hate* not having a load (speaker cab) connected.

(Sorry to state the obvious if you already know this stuff!)

Thanks,
GregL.

[needsmorecowbel]

Cheers Greg perhaps you can clear this up: do you divide the overall cab wattage by the number of speakers in the cab to get the individual minimum speaker wattage? For example in the 60 watt amp I had could you use 2 x 35 watt 10 inch speakers or would both speakers have to be rated above 60 watts? I've looked around on the net for an answer to this but there is so much trolling of forums...

Stu

[Nick]

Before I bought my amp I went to a presentation where a rep from Fender USA (a fairly handy guitarist to boot!) demonstrated all their amp models. One demo (using the same guitar) was of the Fender Hot Rod DeVille (which was the one I finally settled on). Firstly he played through their 2X12 model then through the 4x10 model. To my ears at least, the difference was night and day, whilst the 2X12 was still good, it lacked the crispness/clarity of the 4X10, was a bit fuller and probably would have suited an all humbucker loaded guitar where as the 4X10's had the 'bite' I love. So a side by side would be a good experiment for you Stu now you've separated the two parts to see which combination suits your axe the best.

[GregL]

Hi Stu,

Guitars tend to be tougher on speakers than most other applications. So, as a good rule of thumb, calculate the power handling of a speaker cabinet containing multiple speakers, as:
Total wattage of speaker cabinet = (numbers of speakers) x (wattage of lowest-wattage speaker) eg cab with 2 speakers, 1 @ 35 watts and 1 @ 60 watts then total wattage of cab = 2 x 35W = 70 watts. Further, a "safe" rule of thumb (for valve guitar amps) calls for speaker cab wattage = 2 x amp output wattage eg 60W amp -> 120 watts of speakers.

BUT, the more important consideration is speaker impedance - whether the speakers in the cabinet are wired in series, parallel or series/parallel (if more than 2 speakers in the cab). eg 2 x 8 ohm speakers wired in series = 16 ohms, wired in parallel = 4 ohms. Same applies for combinations of multiple speaker cabinets.

Thanks,
GregL.