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bassman (5f6a) preamp tubes

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Aug 20, 2009 3:36 PM GMT

A tube went out on my bassman clone, and had to change tubes around. Found a couple of old RCA blackplate 12ax7's. Was running v1 to v3 12ax7, 12ay7, 12au7.

Now my setup is 12Ay7, 12ax7, 12ax7. It is tougher to dial in the upper volumes before feedback because the volume control has less sweep than before, but BBQ bob is right as usual. The amp has more spank when you do get it dialed in. However, if you want more distortion, it is best to put the lowest gain tube you are using last v3 position.

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Comments (23)

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Aug 20, 2009 4:56 PM GMT
Barbeque Bob M Replied:

The different manufacturers of tubes will give very distinctive characteristics of their own. I know for a fact, for example, that the sound of an NOS GE 12AY7/6072A with the 5 stars on them sounds very different than an RCA version of the very same tube. The RCA, based on certain years, actually may have more distortion to them than that GE version, beut even older versions of that GE will sound different as well. Checking out different manufacturers of the same numbers, but in different positions on the amp, like in the case of where the 12AX7's are ,may surprise you. Most of the 12AX7's today are really 12AX7A/7025's, which had the treble boosted to give more clarity as opposed to gain, and if you run across a real 12AX7, as I have from GE, that's NOS, made around 1960, the difference may shock you. So, what this means, check out the sounds of the different brands in the differing positions and see what you like. This is true even of the newer stock tubes a well.

Aug 20, 2009 5:53 PM GMT
walterharp Replied:

yes bob, i agree. i got lucky enough to get a big stack of old 12a?7 tubes from an electronics friend, and was able to try a whole bunch. the old rca black plates were the best 12ax7 to my years. weirdly, one of the groove tube 12ay7's, contemporary, sounds really good to me, better than the older ones i tried. btw all tubes tested strong, so old does not mean worn out by any means, but not all were nos either

Aug 20, 2009 7:39 PM GMT
Barbeque Bob M Replied:

One thing you should remember with 99% of the Bassman clone amps is that for the volume and tone pots, they usually tend to be using linear taper pots, meaning that unless the knob hits right on the setting numbers, there's no change at all, wheras on the real deal (which I've owned since 1983), those use audio taper pots, which has tons of different settings inbetween the numbers and often the setting, for example, on the clones at 5 or 6 may be 2-3-4 or anywhere inbetween on the real deal.

My old friend Cheryl Arena's Bassman RI volume setting using the same JT30 mic I have on 5, was about 2-1/2 to almost 3 on my real Bassman, and that's just the volume pot. I had 5 different useful bass settings between 6 and 7, and her RI just had one setting, just on the numbers. The audio taper pots are kinda on the sensitive side, you could say.

Aug 20, 2009 8:44 PM GMT
walterharp Replied:

The weber 5f6a amp kits have all audio taper for volume pots, and there is one in the bass tone position, but not in the treble, mid or presence which are linear. This is consistent with the original fender layouts that are posted on the web.

Aug 20, 2009 9:03 PM GMT
Barbeque Bob M Replied:

That's so totally different than if you bought any Bassman RI types, and the only other amp of this typeI know of for certain that's using audio tapers are the Sonny Jr. 4-10's. Is this Weber thing what you've got?

If you ever come across an NOS early 60's GE 12AX7, get one ASAP, and the first thing you'll notice is that the middle on these tubes are outrageous compared to the 12AX7A types that are common place.

BTW, Do you put the volume control you're not plugged into on 6, rather than the channel bridging? This gives more bottom end gain in a much smoother way than the bridging does.

Aug 21, 2009 2:01 PM GMT
walterharp Replied:

I have never heard of using 12dw7 anywhere but v3. The idea there is that the unequal gain on either side puts the power tubes a bit out of push-pull and gives more distortion. In V1 it would give channel 1 and channel 2 different gain, and may do interesting things with a briged set up. in V2 it would be similar to putting a lower gain tube in there. 12dw7 is one side 12ax7 100x gain, and the other 12au7 20x gain, putting the total gain passing through v2 channel, similar to a 12ay7

The 5751 is a Sovtek at 70 gain somewhere between a 12ax7 and a 12 at7, never used that one.

Aug 21, 2009 6:31 PM GMT
Barbeque Bob M Replied:

That setup for me makes everything sound like mush and will severly lack definition at any volume.

Aug 22, 2009 6:39 PM GMT
walterharp Replied:

i am using the tung-sol reissue power tubes, and they have a nice horn-like ringing tone that i like. dropped the bias so they are running a tad on the cold side relative to what a guitar would like.

I ran 12au7 in position 3 and all voltages checked out ok, and it was in there for at least 50 hours of playing, so in mine at least lower gain in v3 was ok. my reasoning for lower gain tubes later in the line, if you are using lower gain tubes anywhere in the signal chain, and want more overdriven character from the preamp, is that you feed a bigger signal into the later stages, and will be more likely to overdrive/ distort a tube that way. The bigger the signal, the more distoration the tube gets, and with that approach you are bosting the signal early in the chain. I have now flipped it back because what sounds good to me in the basement, does not cut through well in a live situation (cleaner is better in that case it seems)

Aug 22, 2009 6:50 PM GMT
Barbeque Bob M Replied:

The tubes I have in my real Bassman are all NOS stuff. In order from preamp to rectifier,it is as follows:

Early 60's RCA 12AY7/6072 (or GE 5 star military grade 12AY7/6072)
Early 60's GE 12AX7 (not 12AX7A/7025)
Late 60's Phillips 12AX7A
Phillips/Sylvania 5881's (pair) (these sound almost the same as NOS Tung Sols, butare more plentiful and less expensive.
Mullard 5AR4/GZ34 The finest rectifier tubeof this number ever made, bar none.

All of those numbers are the numbers that were stock, no harp mods or anything.

Aug 23, 2009 7:34 PM GMT
walterharp Replied:

yeah, bob, it would be a crime to mod that amp, as well as dropping its value.

joe, good to know that, i won't tempt fate, but the voltages seemed ok, and frying a resistor is not so bad in my amp cause it is point to point

Aug 23, 2009 8:59 PM GMT
Barbeque Bob M Replied:

Just remember, whenever you change power tubes or rectifier tubes to something different than what it calls for, before installing them, make sure the amp gets properly rebiased FIRST or you're gonna be paying big bucks to repair them because you'll fry the amp, and many of these Bassman RI type amps, even more so with the Fender ones, are using printed circuit boards, and tho those are cheaper to produce, the expense to repair them is far greater than the old school, point to point, cloth covered, hand wiring of the originals.

Aug 24, 2009 5:30 PM GMT
walterharp Replied:

that would be another argument for choosing a kit 5f6a bassman rather than the fender RI. cloth covered, point to point wired, the person who makes it can mod to harp friendly, and the total cost is not that different from a bassman reissue

Aug 24, 2009 8:59 PM GMT
walterharp Replied:

I used Weber speakers in mine, 2 ceramics and two alnico, after the approach the sonny amps took

Aug 24, 2009 9:10 PM GMT
Barbeque Bob M Replied:

Those Eminence Legend lower watt versions are more Jensen like than the Jensen RI's are. Finding someone to recone those real ones is, frankly hell, because you have to not only get the correct speaker paper, but also the wiring, know the wiring technique, and also the correct glue and glueing technique as wellor it won't sound right at all.

If you do the point to point wire thing for the Bassman, it is VERY important to remember that the wire used is solid core and not stranded, and it will make a big difference.

The orginal 4-10 Tweed Blues DeVilles actually sounded closer to a real Bassman than the original Bassman RI's at the time did.

Aug 25, 2009 12:29 AM GMT
walterharp Replied:

either of you guys use a stand with your bassman? the place we play saturday has a strange linear dimension with a row of seats down the center, so the sound gets channeled down one side or the other. I want to get the amp up, but am worried that a standard stand will make it tippy, since the amp is a bit tall and top-heavy

Aug 25, 2009 2:16 AM GMT
Joe L Replied:

Put it on a four legged chair. That might distribute the weight a bit better.

Aug 25, 2009 2:40 AM GMT
walterharp Replied:

maybe two, side to side is the problem then

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