forte cab questions (2x12 stereo)

AFAIK, running separate amps to each speaker means the response of each speaker will be less colored by the other, but internal reflections will have some effect. I wouldn't worry about that. Considering the cab design and narrow dispersion of 12" speakers like that, I can't imagine the high end of either will come out of the other side vent. The ~4kHz+ range of each should be isolated from each side, and the inverted ~1ms delayed side vent wave shouldn't interfere with the same freq range front waves of each speaker. It looks like it is designed specifically so the 4kHz+ range of each speaker should disperse pretty wide without interference other than right in the middle of the speakers, starting from ~4' out. Below that range there will be mixing, but that's shouldn't be an issue. I'd say go for it. I like your speaker choices. If you prefer to crank the Presence on the Bassman, you might want the ET65. I think Vox are darker amps when cranked. Someone with experience comparing the relative brightness of the two amps might know more, but you can always use more Tone Cut on the Vox at a given volume if it's too bright through the Retro.

Maybe make a note on the back of each speaker connected to each input with the impedance value and RMS power ratings.

Sure thing. I assume the Bassman isn't 100W? You could also swap in your ET90 if you prefer it. A Klone Centaur is apparently the de facto standard drive pedal for a Vox ACxx, or you might prefer a Treble Booster type for a brighter sound. I wouldn't go for originals because they are way overpriced. There are plenty of clones that sound virtually identical. The Centaur is a very interesting design.

Yeah, I was a bit confused about the amp situation. Cathode-biasing rounds/cleans-up the attack. Might be nice to throw something different in one side. Bassman initially ran 5881 AFAIK, but it might make more sense to just leave one KT66 pair and run something with less bass emphasis in the "Vox" side. EL34 might be worth a try -- the smooth upper-mids might be a good idea with the bright Retro 30, and a dynamic clean sound mixed with a more dirty/compressed sound is a good bet. The KT66 might not dominate the bass and treble attack punch mixed with the El34. You might need to try KT88 if that's the case. If you don't know what to try for EL34, I'd start with the JJ EL34. They are a well-liked "neutral" EL34. The JJ KT88 are also very good. I really don't think NOS tubes are worth it, but sometimes you can find a good deal.

Yes, I'm saying to try the JJ KT88 for the Bassman-ET65/90 side if the KT66 isn't loud enough mixed with EL34 in the Vox-Retro 30 side. If you are concerned that KT88 won't give that sweet Bassman-like midrange sound, try a pair of Tung Sol 7581A. They are a very full/balanced/clean/detailed 35W tube that breaks up smoothly like a 5881 -- very highly reviewed by those that have tried them, and make more sense for a Bassman sound if the KT66 don't have the power and punch. The JJ 6L6GC is clean hard and bassy. The spectral balance is virtually the same as the JJ KT88, but the high end might be "harder" than your KT66 (depending on the KT66 brand you have). The JJ KT66 is like a weak/flat sounding KT88 -- not their best effort. The Shuguang KT66 or the smoother Gold Lion are better. If you want a fat 6L6GC sound in the Vox side, but with tighter bass and the smooth EL34 midrange, maybe try the JJ 6CA7 -- probably the most reliable tube power tube JJ makes. The JJ EL34II are fat like 6L6GC, but "softer" than the 6CA7 and with the stronger 3rd harmonics that pentodes have -- sweet sounding tube, but not as clear as the 6CA7. Technically, might be more Voxy though. Since the attack clarity would come from the Bassman side, they might be the best option. The sweet high end should be nice with the Retro 30. The other option is 5881 in the Vox side -- aggressive midrange and more-compressed/less-bassy than 6L6GC or 6CA7, but tight and gets creamy when driven. I'd think EL34 would be more Voxy though. Some other EL34 brand might be better, but I don't know off hand. My impression is the Mullard RI and other recent Russian EL34 tubes are more mushy than the JJ. That's why I'd try the JJ EL34 or EL34II first. Being an ultra-linear design, I'm not sure how much tube overdrive the amp generates. I guess it depends on the transformer tap values:
https://www.ampbooks.com/mobile/amp-technology/ultralinear/

No problem. I find doing research interesting. I'll just mention a few more things about the tubes I suggested. Power tubes don't make all that much difference until they are driven. Even then it's not that much, but it's worth finding something you really like if it's not a big expense.

Some the TGP forums say the JJ EL34II is "brighter" than the 6CA7, and that it's the "darkest" of the JJ power tubes. Opinions are divided, but they may worth a try at some point. Either tube will sound softer and more-compressed at the low plate voltage of your amp. More mushy tubes might then be too mushy for your taste. I think the 6CA7 is the less mushy, and perhaps less resonant, of the two. They sound really huge in old Plexis without getting muddy like 6L6GC.

The KT66 you have may be close enough to an EL84 sound anyway. You might just stick with those, and maybe just try the Tung Sol 7581A in the other side if the speakers don't quite make the difference you want. I'd get any JJ tubes in matched sets from either Eurotubes or TheTubeStore to maximize odds of getting good ones (any tube from any brand can fail even after testing). TheTubeStore also has the TS 7581A.

The Tung Sol 7581 is just a repackaged 6L6GC. The 7581A I recommend is a better tube, and will work with virtually any amp that takes 6L6GC. You may get ~35W from a pair instead of 30W. The JJ 6CA7 is a direct EL34 replacement with a 500V G2 limit (vs 450V for most EL34). Maybe just try the speakers first anyway.

Update: I looked at the ST88 amp on the Frenzel site, and it lists 30W for KT88 tubes. The JJ KT88 are rated as 600V/42W tubes, so the plate voltage of the amp must be ~250V? I'd guess two 550V/25W KT66 would then yield ~20W in cathode-biased push pull at 250V. EL34 should then do ~22W. A 7581A pair should do ~30W, and keep up better with the KT66/Vox side with the "edge" (No NF) switch engaged. I'm really thinking that's the first tube set to try changing. I'll try to otherwise distract myself from this in future. LOL.

I'd first just try the 7581A in the Bassman side for ~30W. Can't say which EL34 to choose if you aren't happy with the KT66 in the Vox side. I just tried to describe how some EL34 types would sound. Yes, a real KT66 pair will put out ~20W because it is a 550V/25W tube vs the EL34 which are 500V/25W tubes. They also take more gain to distort, so they won't generally sound as loud unless cranked hard. They also don't compress as much as EL34, so they have stronger fundamental note response. In that way, I feel they are closer sounding to EL84 than EL34. Which KT66 brand do you have anyway?

Vox AC amps don't use any neg feedback, so you should engage the "Edge" pull knob on that side for a more Voxy sound. I imagine the volume between the channels will even out when the Gain and Master knobs are up, so I'd think you'd want the extra wattage on the Bassman side for the cleaner attack sound to come though.

Incidentally, I noticed the Harley Benton 2x12" cab is inexpensive and very well-reviewed. You might consider one of those, then just remove the handles and screw/glue in ~45d angle reflecting boards inside the back to get a wide stereo effect. Probably easy to do if you are comfortable building things. You'd save some serious cash.

Makes sense that the amp “needs” the Edge engaged with the JJ KT66 -- they are reportedly rather flat/sterile sounding. It must sound more lively without NF damping them down. JJ tubes are also kinda “hard” sounding. I think it’s a tight vacuum. The GL KT66 are smoother if you feel like spending the money. Otherwise, the JJ EL34II are a soft sounding tube for ~⅓ the price. Possibly the only one without a very tight vacuum. Much more harmonics than the JJ KT66 -- fat and somewhat bright, but not harsh. Maybe try a pair of those. It may be boomy with the Edge on. Turning the bass down should solve that. I'm also curious about the preamp tubes. Stacking different types brands can alter the character a lot. Not sure if the amp has a phase inverter stage, or what type it is. I'd at least put a nice long plate tube in that stage if you have a choice. Specifically what are the preamp tubes?

Yeah, I thought the ET90 might be good for a Bassman sound. I’d still try those 7581A driving it at some point. Probably smoother than the JJ KT66.

I could be mistaken, but I think the Retro 30 is a ceramic BH 50. May not be much difference -- less bass and slightly sweeter high end? My guess is the EL34II or GL KT66 may soften the high end enough with the Retro. You’d have to experiment.

I actually meant which brand and specific type. I assume the L & R "output" are power tubes? Don't Bassman normally have a 12AT7 for the phase-inv? I wouldn't change that unless the socket is biased for it -- may sound too brittle. In any case, the standard JJ ECC83S is considered a fairly dark tube. Either way, stacking a few tube plate types should fill out, open and sweeten the tone, and add some subtle complexity via different plate resonance, rather than the same resonance stacked. Here's what I'd do:

-Try a JJ ECC83MG (Medium Gain/plate-size) in V2 and V3 and/or V4. It's a better (sweeter & more open) sounding tube. The gain difference is only ~5% less than the standard ECC83S.
-Try a JJ ECC803S (long plate) in V5 & V6. You could instead try a Sovtek 12AX7LPS in either slot. It's a more mushy and harmonicy tube some players prefer. For that money, you might look for a long plate IE 12AX7 if you can find one cheap. Do not use the long plate "spiral filament" tubes in cathode follower slots. They can't reliably take the cathode voltage.

Those four tubes should total $60~75. Not that much difference, but you may notice some subtitles like cleaner bass and better high end detail. It certainly won't make as much difference as a speaker change, but more a matter of fine-tuning the response character. You could try recording b4 installing them, and again after to see if you can hear it.

AFAIK, cab size/volume does affect the bass response via air compression and where the cab resonance freq is. Both cabs are essentially open back in this case, so compression isn’t really a factor. The Fs (Free Air Resonance) of each speaker cone should be the dominant factor in bass response, around which point it will be louder. Considering the relative size of the openings, both cabs may be tuned about the same. So the difference in internal volume might end up causing something like a 0.5dB or less difference in bass response above the cab resonance -- which would be in the mid-bass region. My guess is that difference will be insignificant compared to the bass response traits of the speakers.

Since the Forte cab projects the high end from each cone back forward, effectively dispersing it where you’ll hear it, it should have a better stereo effect -- probably over 2x wider above ~2kHz (where stereo imaging really starts to matter). The cab would still need to be tilted back or placed up near ear-level to hear ~4kHz+.

I assume both master volumes are up the same? Both Edge switches set the same?

Be better to drive the EF806 harder. I assume the tone stack is configured after it. Try cranking the Middle knob way up and adjust the Bass and Treble for tonal balance. The Middle knob is what really drives the PI/amp section. Maybe you can add one 12at7 or 12au7 triode stage b4 the EF806? Might not be as hard as you'd think if the power transformer can handle it. One of the first 12ax7 triode stages in the Bassman may be unused, or you might be able to split it to form a new Vox input? Otherwise, you might just try a JJ 5751 in the first Bassman slot (V2?). That will make some difference without really altering the tone, but I think the JJ 5751 is a better sounding tube than the ecc83S anyway. The other preamp tube changes I mentioned may help too. Cheaper than a new pedal in most cases, and I think you'll prefer the results. Those TS 7581A power tubes should be almost 2dB louder than the KT66 at the same Voltage. That would bring the level of the Vox side up. Incidentally, most seem to agree that the Shuguang KT66 are the best sounding new KT66, but see what you think with the better preamp tubes first. Inserting a SS pedal into the FX loop will defeat the V4<->PI tube impedance interaction. You may notice a slight tone/response change.

You don't really need to use lower gain tubes If you have separate power settings for each amp. I had intended to recommend the 5751 for the Bassman driver, but I'd try an ECC83MG there if the amps have separate power settings, unless you want significantly less gain.

Yes, you can substitute a 12at7 or 12au7 in 12ax7 slots, but the baising won't be right as is. A 12au7 might be too grainy and unbalanced sounding. A 12at7 will be cleaner and have slightly more high end. You might like one in the Bassman PI slot if you want less preamp to power amp gain and higher sparkle.

V5 & V6 are PI slots, right?

AFAIK, You can't overdrive the V1 EF806 unless you drive it with a pedal or something. It's a high gain pentode that does not drive back through it self like a dual triode can. The Bassman preamp probably has more total more total gain because of the extra triode stage in the first 12ax7, the position number of which I don't understand from your descriptions. Yes, there is nothing risky in cranking either preamp gain up if you want, but you'd need another internal tube stage to overdrive the EF806. Diming either amp will break up the amp section, especially with the low voltage point they run at. It should be even more noticeable with the Edge switch engaged.

An ET65(or 90) or Retro 30 will not start to break up until at least near their RMS power rating. If the amp section break up is too crunchy than you expect, it's probably a combination of short plate PI tubes and the JJ KT66 power tubes. The JJ KT66 are more like KT88 -- great for metal, but not like the original GEC KT66 at all. Do you even know if the power transformers can supply the full 1.3A for KT66 tubes? It can cause PT overheating if not. I'd try a pair of TS 7581A to compare either way. You might need those or some other 0.9A-rated 6L6 type with your PT's.

To break in your speakers, you'll need to crank the those amps near full power for many hours. There's a way to use a Variac if you want to buy the ~$70 Chinese one online. Run it at ~1/3 the rated speaker voltage on either speaker out of the cab and leave it overnight. Look it up for more details, and use with caution. Hope we get to hear some clips at some point.

Your tube purchasing recap is all correct. It should total ~$150, and the amps will be deeper with more low mid-emphasis and a smoother high end.

You might prefer a more mushy Sovtek 12ax7LPS in the PI of either amp, but they are more expensive. Maybe try one later.

The JJ ecc83MG is very low noise and has a more open high end than the short plate ecc83S. You may still prefer the darker/more-compressed ecc83S in the first or second stage of either amp depending on preference with a given guitar.

I'm wondering if the EF806 might be running in triode mode? That would substantially reduce output and dynamics.

Power tubes don't have to be that closely matched, but matching doesn't hurt.

Preamp tubes don't need matching because they run in class A "series" mode. Balanced triodes are good for the PI stage for the intended performance, but are not necessary. It doesn't cost much more anyway.

I'd buy the JJ's from Eurotubes (for the best testing procedure), and get the TS 7581A pair from TheTubeStore as a matched set.

Last big tip is to use a low capacitance guitar cable for smooth extended highs. ~200pF can be good for a lot of things.

Yeah, you need to be within ~20d on-axis to a 12” guitar speaker to hear the 4kHz+ chime, so see what you think when it’s up closer to ear level -- on the arms of a folding lawn chair like this?:
https://express.google.com/u/0/product/4981298279921624995_1797322828067...

Hanging a rug across even one wall can really help in an concrete room. Do two walls if you can. Across the corners is a good idea.

Preamp tubes can make at least as much difference as power tubes, and anyone I know of that’s tried the ecc83MG prefers them to the ecc83S. Use them in the PI too if the ecc803S is to bassy for you. Either way, I don’t think you can go wrong with those TS 7581A. The JJ 6CA7 are actually considered a relatively dark tube, so it depends what you think about the cranked amp sound when the speakers are up higher. You might even prefer the 7581A in the Vox side. My impression from reviews and demos is the JJ EL34II are more like EL84 than are the 6CA7, but I can’t really say what you’ll prefer...and you might find any EL34 too dark if the amp doesn’t have an EL34 impedance load option. Again, the PT must supply enough amps for EL34 types.

I think the Ruby 6L6GC are ShuGuang. Yeah, makes sense they are more lively, but harsher than the JJ KT66. I think the ShuGuang KT66 are sort of in between those two. You might like those in the Bassman side, but I think the preamp tubes I suggested would open up the sound with the JJ KT66 nicely.

There's really no reason to order a quad of the 7581A without trying just a pair first -- tube matching doesn't apply to separate amps. KT88 might be worth a try, but I'd think the EL34II are more like EL84. They break up sooner than KT88, so they may actually sound louder at mid gain settings due to the the added harmonics and higher level average loudness. The EL34II have a smooth midrange and soft high end, while the KT88 are more crunchy. The JJ 6CA7 is cleaner and darker than the EL34II with a smoother midrange than the KT88. 6CA7 give a massive roar and chunk in a Plexi, but I think the JJ KT77 is the best of all worlds -- fat bass, smooth mids, and sweet clear highs that get creamy when driven. They actually might be the best Vox side option, and an easy output match for the 7581A. Any power tube type will be darker and more distorted & saggy at the relatively low (~300V?) supply voltage, so cleaner fuller sounding tubes might make more sense. You can always set the Bassman side power switch to 1/2 if you need to. Either way, I wouldn't order four power tubes of any type without first trying just a pair of something to compare to the JJ KT66.

Again, you should be able to split the Bassman side V3 driver and rewire the preamp inputs fairly easily if you want. A real Bassman just uses one driver triode per channel, and you'd have a triode with which to overdrive the EF806, for which you'd want an extra gain knob, maybe with a bypass switch. That would take some research, but I don't think it's that difficult. Otherwise, you might just consider a JJ 5751 tube in V3.

If you don't want to spend quite as much on preamp tubes, maybe just order three ecc83MG and one ecc803S from Eurotubes (or The Tube Store) for a total of ~$60. Try the ecc83MG in V2 and V4, and in either V5 or V6, and try the ecc803S in the other PI slot -- my guess is you'll want it in the Vox PI slot to help mellow the high end and fatten mid-bass of the Retro 30. You could even order one JJ 5751 for V3 (instead of a third ecc83MG) if you know you want considerably less Bassman side gain. You can order another ecc803S later if you really want one in the Bassman PI slot. I doubt you'll want to, but you could then use the extra ecc83MG in V3. As I said, preamp tube variants can make as much difference as power tube types, if not more. I'd do that before experimenting with power tubes so you'll have a static reference point. You may even like the JJ KT66 just fine with those preamp tubes.

We are getting somewhat circular at this point. I think it's time to take the plunge and try something based on what we've discussed thus far. BTW, have we established if the amp is cathode-biased? If not, you'll need to rebias for different power tubes.

P.S. I didn’t originally recommend the JJ KT77 for the Vox side because I thought they’d be too clean and bright, but I forgot that your amp has a low supply voltage, so they are now my #1 Vox side pick. Probably the best sounding JJ power tube, and louder than the JJ KT66. Here’s a good comparison to the JJ 6L6GC and Mullard RI EL34 in a JCM800. Listen to how comparatively full, balanced, sweet and loud they are:
https://www.youtube.com/watch?v=G6nhyOctVBU

Here’s a comparison to the more-expensive/muddy GL KT77:
https://www.youtube.com/watch?v=2wQscYSOoWg

The JJ KT77 apparently have a grid leak resistance value limit of 250k compared to 500k for EL34, but KT77 were actually designed to run in a Ultra-linear circuit. I imagine the grid leak limit won't be a problem since your amp is designed to accept all types of octal tubes -- even less a concern if cathode-biased, which I'd think it is.