preamp tubes 12ax7 vs. 12at7

[scoots]

i have a hot rod deville 212 and i've been reading how people are replacing the 12ax7's with 12at7's. i bought a 12at7 (the only one the store had) and started switching it around. in the inverter position (v3) it seemed to suck the life out of the amp. in (v1) position it "hummed?", i thought i blew a speaker until i moved the 12at7 again to it's final resting place--v2. here it seems to take the edge off the "drive channel"- but takes too much. these changes are being made to keep your clean sound before it starts to break up. did i get a hold of a bad tube or should i stick to 12ax7 ----also what about these 5751's?

[scoots]

iam going to bed will check responses tomorrow, g'night everyone

[sjhusting]

12AT7 has a mu (think of it as theoretical gain) of 60, compared to 100 for a 12AX7. A 12AT7 shouldn't "suck the life" out of that amp - before you write it off you might want to try some others. Also, if you haven't done so, rebias the amp (or have it rebiased) - it comes pretty cold from Fender.

In my experience, fwiw, V1 is critical to the response of an amp - if you put the wrong tube in here, it does indeed "suck the life" out of the amp. There is so much variation among tubes, as well as among tubes of the same brand and type, that I wouldn't draw a conclusion about all 12AT7s based on one single tube.

In my Deville, I use a GE5751 (mu 70), a Phillips 12AX7 in V2, and an RFT 12AT7 in V3 (PI). Putting the 5751 in V1 really brought the amp to life. I think Sovtek makes a current production 5751, but I've never tried it. There are more differences between a 5751 and a 12AX7 than just gain - the 5751 seems to be very difficult to overdrive (in a good way) and the triode halves are reputedly very balanced, which may be an advantage as a PI. I wouldn't expect the current production Russian tube to have those qualities.

steven

[JohnnyCrash]

In a newer Master Volume/Gain type amp like a HR I would not bother.

The Master Volume completely eliminates any need to fool with different gain preamp tubes.

I would also interject that amps were designed to run on a specific type tube... but then again, amps weren't originally designed to distort/clip/overdrive :)

Leave the tube type as is and play with tube brands... the HotRod line of Fenders have Gain knobs, whereas playing with 12**7 types was the forte of earlier pre-MV type amps.

Speakers and tube brands is the best playground for amps like yours.

[scoots]

appreciate the input sjhusting and JohnnyCrash, i am new to the tube amp game, but i really enjoy playing! also what is a 12au7 used for? any preferences for output tubes-i was thinking TAD 6L6GC's they're supposed to be close to the 7851's but without the price tag

[sjhusting]

12au7 is very low mu (20) and probably won't work well. I tried one in my SE 6L6 amp to lower the gain, and it sure did lower it, way below what I was looking for. I don't think it is interesting in a Deville.

I have the TAD short bottles (6L6wgc) in the Deville, and they work quite well; it depends on what you are looking for. I hadn't heard the GCs were supposed to come close to the 7581 (which was the American version of the KT66, which was a British variation of the 6L6, which was a cheap version of the 807 ... )

If you don't know Justin Holton's site for the HRD, you should look at it; most Hot Rod questions have been answered there. Just google for "unofficial hot rod deluxe" and you'll find it.

[scoots]

love that "unofficial hot rod deluxe" site. i was looking at that 12at7 (still in v2) and i noticed it's not glowing as bright as any of the other tubes, including the output tubes. i took the 12at7 out of v2 and put back in one of the 12ax7's. this one glowed as bright as the others. all orange with the exception of one of the output tubes which is now getting that blue glow. put back in the 12at7 and still isn't as bright-you can barely see it. if the 12at7 is a lower gain tube is it normal to not glow as bright? thanks---

[JohnnyCrash]

That glow is generally only the heater filament. If a plate is glowing red that usually means something is wrong, but this doesn't sound like your situation.

The blue glow is fine as well.

[sjhusting]

Like Johnny said, the glow is just the heaters, and if the construction of the tube is such that not much heater light gets out, then it's not going to glow as much. The heaters are in the middle of everything else; you've got the cathodes, grids, and plates surrounding them.

Also as Johnny said, the blue glow is normal. I believe it is ionization of the glass envelope. Makes it purty. It's not gassiness or anything else which is not good. Red-glowing plates (the big grey or black pieces of sheet metal that you see in there) is bad; this happens in the power tubes when they have been biased far too hot.

[11 Gauge]

The Sovtek 5751 should be a fine tube, indeed, as it is closely modeled after their 12AX7LPS's, which are awesome, one of the best modern production preamp tubes I've used.

Be careful with the 5751 moniker - often times it can be BS - just a relabled lower gain 12AX7.

The sweetest lower gain 12AX7 variants I have ever heard are old 7025s. Unfortunately, they are silly expensive and not the best idea for an amp like a Hot Rod.

Barring reverb and trem circuits, V1 is almost always the most critical slot. Others are more flexible, or the circuitry surrounding them helps to tame them without having to fiddle with the gain level. V1 is much harder to dial in, even with a MV.

Lower gain V1s in amps with lotsa gonzo like Marshalls tend to sound wonderful...

[scoots]

once again , i appreciate the input. question- is the hot rod deville a one channel amp. i run a boss pedalboard through the clean channel and what i'm looking for is as much headroom as possible as clean as possible. i like the brightness of the 12ax7 in the v1 position, but if i can find a tube that won't break up as soon... i'll try a 5751. is that brightness due to the type of the tube or the brand?, a combination of both? i put the 12at7 back in the inverter position for now, i was pretty quick to pull it out last time, i'll leave it here and play for awhile. also, what are characteristics for failing output tubes? it seems i've changed my treb, bass, and mids to keep it sounding the same - not only after switching around pre tubes, but sometimes just out of the blue. seems some volume loss too, and this was experienced before switching around preamp tubes. the amp was a demo model. i'm not sure what you call it -- string clarity? it sounds "muddy" turned up. thanks for letting this tube amp new guy drain you guys for info, much appreciated gentlemen...

[scoots]

i think i'll leave the 12at7(it's a electro-harmonix 12at7eh) where it is for now. it's a noisy tube although. this tube had the fender logo stamped on it. the 12ax7's are sovtek 12ax7wc's. these do sound nice. these are the tubes that come with the amp.also had the fender logo stamped over. does fender make all vacuum tubes?-haha how many different brands does fender do this with, from what i understand this is what groove tubes does too. g'night everyone-scoots.

[sjhusting]

No, Fender never made tubes. That's a complicated process requiring a lot of special machinery. Currently there are as far as I know the following manufacturers:


Reflektor, Russia (EH, Sovtek, Tung Sol, others) owned by Mike Mathews (EH)
SED (Svetlana), St. Petersburg
JJ (Slovakia)
EI (Serbia) - producing again, though limited
Shugang and an unknown number of other Chinese factories
Western Electric, USA - only a handful of unbelievably expensive Hi Fi tubes (300, etc).
Emission Labs in the Czech Republic - again, hi -end hi fi tubes.

Any others?

Check Justin's site, troubleshooting rubrik, for the volume loss. I have a vague memory this is one of the common ailments of these amps. Muddiness, too, though there are easy ways to combat this (small change to the tone stack, different speakers). The amps put out a lot of booming bass that has to be controlled.

Is it 1 channel or 2? It's "sorta 2 channel." The drive "channel" kicks in an extra gain stage, which is V2a. (I'm doing this from memory). Both "channels" go through V2b. "more drive" ups the value of the cathode bypass. I'd have to look at the schematic to say for sure. So really it's a one-channel amp, with a switchable additional gain stage (also does a few other things) and a switchable boost on the additional stage.

For maximum clean headroom, follow the instructions on Justin's site under "Helpful Hints & Tips." You might think about the mid mod (adds a jumper to the midrange control, reduces boominess) and the reverb knob (removes a highpass filter which makes for harsh reverb). Both are easy, quick, reversible mods. I did them both, and put a switch in to move the reverb back and forth. I also replaced the master volume (25K linear) with a 250K audio pot - a higher value would have been better, but these are non-standard pots in a funky cage, and I bought the highest audio-value replacement pot from Fender. Made the drive channel somewhat useful, before, I avoided it like the plague.

Sorry Johnny, but although this is a far more modern amp than I like, I don't really consider it high-gain like a Soldano or Boogie. The clean channel is quite useful, and it responds well to a good tube in V1.

Having said that, since I built the 5e3, the Deville just sounds lifeless, so I don't use it much anymore. I'm hanging on to it though, as long as I have it. I'm thinking it might be a good platform for one of the new boss/fender pedals. I'll be picking up a FDR-1 one of these days to see how it sounds.

steven

[JohnnyCrash]

What do you mean by "pedalboard"? Is it an effects processor, amp/cab simulator, or a few FX pedals on a board? If most of your sound is coming from FX then you shouldn't be worrying so much about V1 swapouts.

CRAP READING MATERIAL:
Leave the phase inverter tube with what it wants. That "unofficial hot rod deluxe" page is misinformed in some aspects. Its a PI for Pete's sake not a cascaded gainstage (which according to this unofficial page means its a compromised design for both Clean and Dirty).

I'm also uneasy with his suggestion to clip cathode bypass caps "one by one" until you're happy with your tone... WTF

It is a true dual channelled amp, this unofficial page confuses shared tonestacks for channels. Cascading a gainstage for overdrive is something different. This 60w amp should be very clean at very high volumes, why are you still trying to decrease gain? Learn to use the volume, drive/gain, and tone controls before swapping out tubes.

DESIGN:
Amps are designed to use certain tubes and often sounds best when using those tubes. I would generally only swap out the V1 position when trying to "tweak" tone (and usually only as a last resort)... and at this point its generally in volume and/or gain (overdrive). This is pretty much a one-way street as the tubes you swap out for that 12AX7 will DECREASE in gain characteristics.

IT AINT BROKE, STOP TRYING TO FIX IT:
A 60watt 2x12" SS rectified 6L6 based amp should be PLENTY clean and loud... forget all of that crap you read and get to know your new amp FIRST. Tube amps may not be what you're used to, so try to get used to em before following stuff you read on the 'net (isn't it ironic this very advice I'm giving is on the net? HAHA).

If you need to decrease Gain make sure your Drive select switch is not engaged, your Drive control is turned down, and you're in the Clean channel.

You mentioned Volume loss, string clarity, and muddiness in your post. Something may be WRONG with your amp.

This amp should be plenty clean for you, unless you're playing an arena unmic'ed.

[sjhusting]

The amp is loud and clean. Real loud. Replacing V1 is an easy experiment. I did the other mods to get what I wanted out of the amp. I don't like messing with those pcb amps, you can lift traces too easily. You're right about the volume drop - take it to a tech. Again, that PCB makes it a hairy proposition, and there could be too many causes for the drop to just muck about looking for it.

But I'm looking at the schematic, and it sure looks one-channel to me, with an additional stage. Not that it matters.

Now please stop yelling. I don't think there's any disagreement here. Where's the harm in trying out a couple of different tubes? Maybe it will bring it closer to what he wants.

[JohnnyCrash]

Originally Posted by sjhusting

Now please stop yelling. I don't think there's any disagreement here. Where's the harm in trying out a couple of different tubes? Maybe it will bring it closer to what he wants.

Sorry if it sounded like that.

I am not yelling, but rereading the post I can see how it looks a little... I guess "hostile" is the word.

Again, sorry if it looked angry/hostile/yelling... NOT my intention at all.

I guess I could've summed it up better too:
As he said he was new to tube amps, I say: get to know the amp first. This will help you understand if you need to do anything arbitrarily read online or if something is "normal".

CHANNELS:
As far as the channel comment, I was merely saying it is not "compromised" on either Clean or Dirty "channels".

1. Getting into semantics, Fender says it explicitly has three "channels." Many other amp manufacturers use this language (the Peavey Classic series for example) and it is not incorrect or immoral for them to call them "channels."

2. Getting technical, its more complicated. Some folks say "true" channels have to be entirely seperate topographical preamp sections. Other folks say things as simple as cascading gain stages is a channel... I guess it depends on what school of thought you subscribe to.

For all intents and purposes the way Fender designed the "channels" does not "compromise" anything on this amp, as that site asserts. This amp is VERY capable of staying VERY Clean on high volumes. It is also capable of higher Gain sounds.

Although - everyone's taste in overdriven tones is as varied as SS amps, OD pedals, Diode Clipping, and "pure" tube... BUT the overdrive's character is not the user's complaint with this amp - it is headroom of the overall amp.

If the amp isn't "clean" enough, I suspect:
1. Something is wrong with the amp.
2. The user is not used to tube amps.
3. The user doesn't fully understand the Drive/Volume controls.

Either way, we can all agree that this 60w beast should be very clean at high levels of volume.

Again, I am sorry if my post appeared to be hostile in any way.

[sjhusting]

Johnny, no problem, like I said I think we agree here. So this is my last word:

1) he needs to take it back to get the volume problem looked at.
2) you are absolutely right about "getting to know his amp." I modded mine, but I've been using tube amps for 30 years and know what I want.
3) The deville - mein anyway - can be VERY VERY LOUD and VERY VERY CLEAN, stock, without mods
4) I haven't actually read anything on that site for a long time, so I'll take your word on the "compromised" business.
5) You're right about "channels." I don't view an inserted gain stage as a "channel," I look at it as a switchable boost. For me, a 5e3 has two channels, because the input goes through separate triode halves, neither of which is used by both channels. In the HR series, the input goes through v1a-v1b, v2a is switchable, then it goes to v2b. The more drive does some capacitor stuff.
I don't really care though, because, actually, if you want a second channel with more gain in the form of an additional gain stage, then this seems like a pretty reasonable solution, it requires the fewest additional components. If you wanted a BF and a Plexi channel, it would look very different. I like my amps simple (which a HR is definitely not) so I prefer vol-vol- shared tone stack without much circuitry. Someday the insides of my Deville are coming out and it will become a tweed Bandmaster or something. But right now it is the only amp I have that really does REALLY LOUD CLEAN, though I have to put a linear booster in front of it for some reason, which I don't need with the tweed.

I'm going to shut up now, since I am relatively new to this forum and never intended to talk so much. If you're ever in Germany stop by, I'll buy you a beer.

steven

[JohnnyCrash]

Originally Posted by sjhusting

2) you are absolutely right about "getting to know his amp." I modded mine, but I've been using tube amps for 30 years and know what I want.
3) The deville - mein anyway - can be VERY VERY LOUD and VERY VERY CLEAN, stock, without mods

Its all good... I like talking about amps and all of this fun stuff :)

I think this may be the problem... guys like you and I are very used to tube amps and this type of inquiry on preamp tubes may be clouding the issue of a troubled amp... we know what we want, even if its minor, scoots is new to this amp.

Between all of the posts, I'm sure scoots is going to have a fun journey discovering his new amp's sounds.

Originally Posted by sjhusting

I'm going to shut up now, since I am relatively new to this forum and never intended to talk so much. If you're ever in Germany stop by, I'll buy you a beer.

steven

Talking makes this place great! I enjoy talking about amps with you :)

Besides, if clean headroom is the trouble and the amp "out of the blue" suffers from volume loss, lack of clarity, and mudiness - all of this discussion and debate may have uncovered a real technical problem with scoots' amp that needs to be fixed.

German beer... I hear it is very tastey... I may need to make up an excuse to go to Germany! If you ever make your way down to Southern California I'll crack out some Kentucky Bourbon for you!

[scoots]

i think i may solved the volume thing, one of the 6l6b's were loose. i did bring it in at one point, but the guy at the store had run it all day next to the store model and said they preformed the same. i hope this was it, i haven't had any problems since. also, don't get me wrong guys, i really like the sound of this amp, the clean is like---butter! i just want to see if i can get "more", whatever that may be. maybe it's like a "grass is always greener... or the tubes are always brighter in the other guys amp". sj husting, JohnnyCrash-thank you, this is exactly what i was looking for. solid arguments on both sides of the coin. gentlemen, make your way up to minnesota---i'm buyin'!

[sjhusting]

watch what you ask for i have friends in Mankato and a sister in St. Paul ... ever hear of City Mouse? Or the Castaways?

steven