Tube swap

[ranjam]

........ don't go snipping bits and pieces in your new amp if you're not 100% comfy with doing so..

As Charlie Chan would say.... Egg-Zachary. To repeat myself; the 'brite' circuit is that cap you could snip, if you felt up to the task. But again, '10' is '10', and no effect would be heard. At say '7' on the volume control, some may call it 'duller', some may call it 'warmer', but it wouldn't be as 'trebly'. At lower volumes, the highs go through the capacitor and not the volume control. You do this all the time on your guitar. But '10' effectively shorts out the treble peaking circuit, so the full range goes on to the next tube.
If you always play the amp on '10', then this is a moot exercise. But I'll back pedal and say take it to a tech. It can't be more than an hour of his time; 1/2 hour to snip out the brite cap. An hour to pull the chassis and bump up C1.

 

[Milnoque]

There are very high voltages in tube amps and they don't always go away when you turn them off and unplug them. Fortunately it is easy to make them safe to work on. IMHO you would do well to get a book on the subject and at least go through the safety warnings. Or you can find out what you need to know with the magic of Google. Please don't let this scare you away. Tinkering with amps is seriously fun. Heed my caution and then try Ranjam's suggestion.

By the way, Do you have another amp with the same speaker impedence? If you do try your amp through that speaker. Speakers can make a big difference.

 

[TheBones]

I have to admit the idea of rooting Around in there is a bit intriguing. I would really like to be able to service and modify my own gear. I just don't know where to start. I found a page called ax84.com and that seems like the most in dePth resource I can find on the net, but a good 85 percent of it is Greek to me. More like a chemistry class as oppose to "a capacitor does this, a circuit does this..." I suppose I've got my homework cut out for me if I'm interested in doing any of that sort of work myself.

 

[Greg_L]

I have to admit the idea of rooting Around in there is a bit intriguing. I would really like to be able to service and modify my own gear. I just don't know where to start. I found a page called ax84.com and that seems like the most in dePth resource I can find on the net, but a good 85 percent of it is Greek to me. More like a chemistry class as oppose to "a capacitor does this, a circuit does this..." I suppose I've got my homework cut out for me if I'm interested in doing any of that sort of work myself.

I'm with ya there. I'd love to tweak and build amps, but alas the electical shit just doesn't jive with me. I had an amp earlier this year that was giving me fits and I was ready to dive in with resistors and capacitors and all that shit, and an up-to-date life insurance policy, but I got rid of it and got something else that didn't need any work.

 

[miroslav]

I have to admit the idea of rooting Around in there is a bit intriguing. I would really like to be able to service and modify my own gear. I just don't know where to start. I found a page called ax84.com and that seems like the most in dePth resource I can find on the net, but a good 85 percent of it is Greek to me. More like a chemistry class as oppose to "a capacitor does this, a circuit does this..." I suppose I've got my homework cut out for me if I'm interested in doing any of that sort of work myself.

Your amp sounds similar to an Epi Valve Junior Head...it (one 12AX7 and one EL84)....but if you want to learn mods, the VJH has been modded to death, and there's a ton of mod info all over the internet, so you can easily follow what others have already done (some guys post up step-by-step insturctions with pics)....plus the VJH is so inexpensive, that there isn't the fear of screwing up something that you paid a lot for.

 

[Milnoque]

I have to admit the idea of rooting Around in there is a bit intriguing. I would really like to be able to service and modify my own gear. I just don't know where to start. I found a page called ax84.com and that seems like the most in dePth resource I can find on the net, but a good 85 percent of it is Greek to me. More like a chemistry class as oppose to "a capacitor does this, a circuit does this..." I suppose I've got my homework cut out for me if I'm interested in doing any of that sort of work myself.

I really encourage you to do this if you have a mind. Here's another site that I found useful when I got interested:

Music Electronics Forum

You really don't have to be an electronics technician do get started. Just keep reading, It will start to make sense.

Good Luck

 

[ecc83]

Single ended valve amps ARE harsh!
Push pull op stages cancel out a good deal of distortion and the overdrive sound is more "progressive".
At end of the day the SE valve amp is just the arse end of a cheap 1950's radio set.

One dodge that was used to clean them up was a CR series network across the op transformer. Try 0.022mfd@630V min (22nF) and 1k for starters. Negative feedback was also employed but this is of little use once the amp is driven into non-linearity.

Beware the ad hoc swopping of the 12AX7 for the 12AT7, you can burn out anode loads.

Dave.

 

[miroslav]

Beware the ad hoc swopping of the 12AX7 for the 12AT7, you can burn out anode loads.

How can you tell when it's OK and when it's not....any obvious signs of a possible mismatch without having all the schematics in front of you?

 

[ecc83]

How can you tell when it's OK and when it's not....any obvious signs of a possible mismatch without having all the schematics in front of you?

You can't. The fact is the 81,82 and 83 triodes are not electrically interchangable. The pinouts and the heater ratings are the same but the surrounding components SHOULD be as per each valve's design rating. I know people swop them with gay abandon but to me it is little short of circuit "bending"!

Dave.

 

[teamodave]

I you like the cleans of this amp but aren't happy with the harshness when cranked, why not try it with a an overdrive pedal that you do like? Maybe a Tubescreamer or something. If you can't get the tone you're looking for, you could always sell the pedal and you're not out very much. Good luck with your search.

 

[miroslav]

You can't. The fact is the 81,82 and 83 triodes are not electrically interchangable. The pinouts and the heater ratings are the same but the surrounding components SHOULD be as per each valve's design rating. I know people swop them with gay abandon but to me it is little short of circuit "bending"!

In my tube swaps....I usually stick to the choices that are recommended in the manuals or by the manufacturer, so I assume they know their circuits, but like you say, the 12Axx family gets swapped a lot, and I have tried the entire family on an amp or two just to see the effect...but for daily operations, I stick to the design recommendations, and when you go too far off (like a 12AU7 in place of a 12AX7)...the amp loses it's personality altogether.

The one tube that's rarely stated as a *specific* swap (for reasons you already mentioned) is the 5751...but I use it in a few of my amps, usually in place of the V1 12AX7, and have had only good results with them, and some of the amps have had them for awhile now with no ill effects. If it's "bending" the circuit, it seems to be doing it in a good way....so far.
I even asked a couple of the designers about the 5751, and while it's not in their specs as a choice, they all basically said, "yeah, give it a try...if you like it, use it".

 

[TheBones]

Single ended valve amps ARE harsh!
Push pull op stages cancel out a good deal of distortion and the overdrive sound is more "progressive".
At end of the day the SE valve amp is just the arse end of a cheap 1950's radio set.

One dodge that was used to clean them up was a CR series network across the op transformer. Try 0.022mfd@630V min (22nF) and 1k for starters. Negative feedback was also employed but this is of little use once the amp is driven into non-linearity.

Beware the ad hoc swopping of the 12AX7 for the 12AT7, you can burn out anode loads.

Dave.

i had a suspicion that i might not be able to swap them out all willy nilly but i had read someplace on the internet that the 12at7 was a compatible swap for the 12ax7. and everything you read on the internet is true right??? as for the 0.022mfd@630v min (22nF) and 1k for starters, over my head. this is my first little tube amp and i just bought it more or less for experimentation. I appreciate any advice very much. might have to do some googling

I you like the cleans of this amp but aren't happy with the harshness when cranked, why not try it with a an overdrive pedal that you do like? Maybe a Tubescreamer or something. If you can't get the tone you're looking for, you could always sell the pedal and you're not out very much. Good luck with your search.

i've tried a few distortions in front of it. a rat, ds-1, a big muff and a yellow drive. so far i've got to be honest and tell ya that the ds-1 seems to be in the lead right now. but , i'm really just trying to be being able to work the cleans through the Kustom. that's why i was thinking about adding a master volume and a tone knob of sorts but….. i've to a little orange that i'm going to a/b for distortion anyway.

 

[ecc83]

" 0.022mfd@630v min (22nF) and 1k for starters, over my head"
Oh? Sorry. Well get a 630 volt foil capacitor value 22nF and wire it in series with a 1k 1watt resistor and connect one end of the network to the EL84 anode and the other end to the transformer HT connection. This will give a "step" down in the HF response and thus filter out some of the harsher harmonics. You can play with other values of cap' and R to your hearts, but make sure the cap is rated 630volts or higher. Some WEM Dominators had such a network but they of course were "proper" push-pull amps!
Dave.

 

[ranjam]

Oi vey! Usually I let harmless tall tales go, since 1) I am not a good debater, and I always get my ass kicked, and 2) they're harmless. Hence the name. And I earlier let it go that using a 12AT7 for a 12AX7 would burn out anode loads. It cannot, but what harm does letting that one go do? However, explaining a conjunctive filter with wrong values could cause harm to a good amplifier.
The filter does go across the primary of the transformer, or for the Defender, the blue and red connectors where blue is the Plate lead and red is the B+ feed (between R12 and R13).
The OPT can experience 'flyback', and the inductive voltage will look for a handy discharge path. This will be a huge voltage, so if you want to use a conjunctive filter, a 630VDC capacitor won't last, nor will a 1-watt resistor. Try a .022uF capacitor, but look for a 1600VDC rated one, and use a 5.6K resistor, but a 5-watt unit. Using 1K will reduce the overall output volume. Heck, 5.6K reduces volume, but some people will accept this. I doubt you'd accept the loss from a 1K resistor.

 

[ecc83]

Oi vey! Usually I let harmless tall tales go, since 1) I am not a good debater, and I always get my ass kicked, and 2) they're harmless. Hence the name. And I earlier let it go that using a 12AT7 for a 12AX7 would burn out anode loads. It cannot, but what harm does letting that one go do? However, explaining a conjunctive filter with wrong values could cause harm to a good amplifier.
The filter does go across the primary of the transformer, or for the Defender, the blue and red connectors where blue is the Plate lead and red is the B+ feed (between R12 and R13).
The OPT can experience 'flyback', and the inductive voltage will look for a handy discharge path. This will be a huge voltage, so if you want to use a conjunctive filter, a 630VDC capacitor won't last, nor will a 1-watt resistor. Try a .022uF capacitor, but look for a 1600VDC rated one, and use a 5.6K resistor, but a 5-watt unit. Using 1K will reduce the overall output volume. Heck, 5.6K reduces volume, but some people will accept this. I doubt you'd accept the loss from a 1K resistor.

I do not say a 12AT7 WILL burn out anodes loads, merely that they can and it has happened.
I did a test not two weeks ago and fitted a T7 in place of a X7. The anode current increased to the point where the 100k resistor dissipation reached 0.46W. That is perilously close to 1/2 a watt and that was with a 292 anode supply. Many older amps run pre amp HT well past 300volts and good design dictates that circuits must tolerate mains +10% forever (253V here) and many amps do not fit even 1/2 watt loads. I agree that for 220k loads there is no problem.

The snubber circuit was used, as I said in the Dominator. I have a Mersey Super, a Dommy clonealike, complete with a decades old cap (could be a 400V mustard, have not looked in ages) bet that is nowhere near 1600volts! Old radios and TV op stages used the network as did the field amp circuits in valved TVs,(about a 5watt 50Hz sawtooth amplifier) do not recall many failures.

The network with a 1k in series will have a Z of about 7.5k at 1000Hz and far from CAUSING a flyback pulse it will act as a load should the speaker be disconnected, that's what snubbers do...Indeed it is transformer winding capacitance that saves their A when such *t'appen. (but don't count on it!).

Dave.