Guild Thunderbird Amp 2X12 Speaker Info

Hello Everyone
A couple days ago I dragged home a Thunderbird. Literally! Man that thing is heavy.
You may have seen it on the Anchorage Craigslist. Looks a lot better than the one picture.
Anyway, there is a better looking one on the Atlanta Craigslist.
I can't type the address cuz my colon thingy quit working.
The picture of the speakers look exactly the same but mine do not have the info on the bell. Missing metal disc maybe. The description says they are Jensen P12N speakers and was wondering if you all might happen to know for sure.
From the info I got from you guys, my guess is the amp is a second generation and has the PC boards. About 1968 or 69? Everything works great. Tubes appear all original and the amp is pretty quiet.

My speaker numbers are 220645 indicating Jensen made in 1966? and 45th week, Right?
Would 1976 be too late for this amp?
The other numbers if they help are EM1250 and C8481

So are they in fact Jensen P12N Alnico or are they Ceramic.
Any advice on recapping?
I am having my 1979 Fender Deluxe Reverb recapped now and my Tech says they used PCBs capacitors back then.
Thought to just get it done after I get my Fender back and be done with it.

Glad I picked up on it. The amp sounds GREAT!
Thanks a lot for any Info
RT

Hi RT: long time no post. Yeah; next time see if they seller will throw in a hand-truck. On the speakers:

Source: Weber Speakers via TDPRI see also Audio Asylum

"Often the Jensens sold for musical-instrument reproduction (that weren't labelled with the amp-maker's name) were in the "Electronic Musical Instrument Loudspeakers" line - the 'EM-XXXX' series.

In 15's, there were (according to my 1966 Jensen 'instruction note' # 245): the EM-1500, EM1501, EM1502, EM1520, and EM1550 drivers. The last 2 models had an alnico-5 magnet, so your speaker is presumably among the first 3 models (?).

In Jensen 12's, in the same "EM-xxxx" series, there are listed 5 models, ALL with ceramic magnets : the EM-1200, EM-1202, EM-1204, the EM-1220, and EM-1250. Actually, there were also EM-1201 and EM-1203 models, but those have tiny [10 or 11 oz.] magnets and aren't what you want.

The EM-1200 and EM-1202 also have 'modest-size', 16-oz ceramic magnets .... kinda "puny", IMO. Really, only the EM-1220 ["for string bass or bass guitar"] or EM-1250 ["for guitar and accordian"] are worth considering/using, IMO. The former has a 60 Hz resonance, the lattre a 90 Hz resonant frequency.

Both are rated for "100 watts" but that has to be 'music power'/peak - not a "rms"/continuous power rating ! Seems you would want the EM-1250 for guitar.

model ....Mag wt ...Impedance VC diam
EM-1220 1 3/4lbs ......8 ...........1.5" ....Bass Guitar and String Bass
EM-1250 1 3/4lbs ......8 ...........1.5" ....Guitar, Combo Organ and Accordian

If your speakers are crinkly battleship grey with a shallow bells then info above applies to your speakers; they have ceramic, not alnico, magnets. In the mid-60s, the US government declared cobalt to be a military strategic resource and prohibited its use for anything other than DoD applications. Nearly all the speakers from the mid-60s on were/are ceramic magnet speakers. They are not Jensen P12Ns unless they have P12N painted on the lip; the 'P' indicates alnico and 1966 sounds right.

Caps; if you want you amp to sound its best, you ought to get it recapped. They won't last forever and any one of them could fail tomorrow or five years from now.

If this is the right amp, the thing on the far left is a multi-section can cap; 2 or maybe 3 sections, typically 40uf at 450V; drop-in replacements are no longer in production; some 'creativity' is required.




This is the power supply chassis; the three cylinders on the right are filtering capacitors; two of them are single-section 250-300uf/450v and the other is a 3 or 4 section cap - all electrolytic.




Finally, your amp has a bias supply and it has its own filtering caps; in this amp, they are underneath. The brown cardboard cylinder is a two-section electrolytic cap, typically 100uf/100v and 50uf/50v. If you get the amp recapped, these need to be replaced too.



I don't know what tech support costs where you are but $200 plus another $100 in parts isn't out of the question. It's the biggest reason people dump these amps but if you get to where you like and want it to sound like it just came out of the Guitar Center ... you know what you have to do. Not sure I understand your man's remark; your amp does have pcbs; printed circuit boards and there are capacitors mounted directly on those boards however the caps mounted on your pcbs are not usually changed as a matter of routine maintenance like the caps I pointed out above.

Good luck; I hope you get your colon fixed ... remember the one about the constipated mathematician...? CJ

Hi Cap
Yeah it has been a while.
Thanks a lot for all the great info.
I suspected they were ceramic speakers. Atlanta guy typical infomercial. He should be on Ebay.
Anyways, glad I didn't have to have it shipped.
The amp actually belonged to a friend of mine who has a ton of cool amps and guitars. Played around with his 54 Fender Tweed while I was there.

Anyway, My preamp section looks bigger and has two of those big silver cans hanging upside down. It has the 5 green PC board in it.
I finally worked out my camera situation so I'll take some pics.
So far its sounds real good, but I'm sure it will sound better. Definetly different than my Fenders. I threw out the Eminence that was in the Deluxe Reverb and put in a Jensen P12N
reissue and made a big difference.
My Fender recap will cost around $170, so I might give it a month or so before doing the T-Bird. Give the guy time to look for parts. Looks like more work for the Guild.
The original speakers seem to be holding up so will see how they sound after the new caps are in.
You guys seem to like the Webers better than the Jensen reissues. Any recommended model?
What was the third generation T-bird? The Maverick was about the last Guild amp model I think around 1971?
Just wanted to say thanks again and will check back later.

RT

Hi RT: Without pics, I had to guess at which model you have. I'm pretty fuzzy on the production dates and models for the later Thunderbirds, Thunderstars, and the rest of them. The Mavericks were medium-power / 7591 amps +/- 30 watts and maybe the last of the combo amps. They did make some brutes in heads; Quantum, Superbird .. amps running 6550s ... beasts! The DR will be easier / cheaper (less time-consuming) to re-cap than the Guild because, from the early '60s Fender made it standard practice to use individual - not multi-section can - caps for each section needing supply.

The caps were mounted in a 'dog house'; take the cover off the 'dog house' on the tube deck and all the caps are lined up there next to each other - one of the many reasons for the enduring popularity of Fender amps among players and techs. Guild and other manufacturers using cans made it a practice to connect the various resistors used to reduce the voltage to separate sections right on the bottom of the can. Removing the can means removing and / or replacing all the resistors; yes, it's time consuming and what usually drives the cost of recapping.

Speakers ... are a little like cheese ... some like Swiss, others prefer Sharp ... but if you want your amp to sound like it was supposed to sound like - what it sounded like when it came out of the factory, you might want to stick with ceramic. As you read above, the Jensens you have were commerical replacement speakers; as good as OEM units ... just not sold in large discounted lots to manufacturers. I personally like Webers because they make alnico speakers that are perfect for low-power, 6V6 vintage amps - the kind of stuff I mess with. I have never had the pleasure with their ceramic speakers but no reason to think they aren't just as good at capturing 'that sound'. Anyway, if you cruise their site, you'll see that they make several models; not just dupes of 60s stuff which I think is really their claim to fame.

A word to the wise; that P12N you mentioned - if it's an older model, it's only rated at 20-30 watts so whether your amp has 7591s or 6L6s, it's at or over its power-handling rating.

Good to hear from you again! CJ

Hi RT66,

It sounds like you have a 2nd generation Thunderbird. The front panel should have the large master volume control similar to the 1st generation Thunderbird. Also the reverb pan will not be mounted under the preamp. In keeping with the ist generation Thunderbird the reverb tray will be mounted inside and on top of the preamp chassis. The 2nd generation Thunderbird still kept the 7591A power tubes like the earlier Thunderbird. The 3rd generation Thunderbird amp is a significant departure from the earlier designs. The 3rd generation Thunderbird amp is equivalent to a 6L6 Superstar amp with 2-12's.

I find the 2nd generation Thunderbird amp to be the best sounding of all the Guilds including the Maverick. The 7591A breaks up nicely and the larger cabinet for the 2-12's adds a very full sound. I tried a lot of different speakers with this amp. The amp's power is high enough to push the Jensens into distortion. I found the distortion of Jensens tended to hide the agressive breakup of the 7591A so I have been using one JBL D120 and one K120's to get the sound I like.

These, and many other Guild amps, are really heavy. It is not uncommon to spend well over $100 to ship one of these babies. A nice 2nd generation Thunderbird amp out of RI sold last week and a mint 2nd generation Thunderbird amp sold out of GA in October. That was a rare occurance of amps. An now one in Alaska! The planets must be alligning!!!

Interestingly Guild used a combination of printed circuits and point-to-point wiring on these amps but reverted to all point-to-point wiring in the next generation of amps.

M

Hi Cap and Mat
Thanks for the extra info.
I just got back my 79 Fender Deluxe Reverb and dropped off The Guild T-Bird tonight.
I took pictures while I had the parts out so will try to post them tomarrow.
My amp tech is having a little trouble locating a 2nd Generation Thunderbird schematic.
Anybody have one laying around?
The power tubes are RCA(Westinghouse) 7591A. Most of the other tubes are original or US made replacements.
The two wire power cord is a shocker if not plugged in the right way. The Death Cap is shot for sure. Got a good 118V buzz holding a guitar while turning on another amp. :shock:

Hey Mat! Are your sprakers originals or reissues. "K" = Weber?
If only made from 68-69, there must not be a whole lot of T-Birds on the planet let alone working ones.
Glad to have one of them. Even as is, It sounded Great. Don't plan on moving it much so might have to double as a plant stand.
Mine appeared to have beed roadied a bit as the top handle is broken, the middle one is missing, and the bottom one is a replacement. The tolex and front grill are still in great shape though.
The Fender DR Rocks!
RT

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Hi RT; send me a PM with your email address; I'll send you the two schematics I have ... although yours is the model in middle. For discussion purposes, I have a 'version 1' schematic; banded metal sides, typically beige, 2X7591 and a 'version 3': labeled 'Thunderbird-Superstar' by Guild, 2X6L6, power chassis hung vertically inside left of bottom cabinet. Good luck! CJ

Hi RT,

My amp came with the same speakers as yours. I swapped them out for a JBL D120 and a JBL K120.


Also, from the posted pi cture it looks like you are missing a piece of aluminum that covers the mounting scrwes to the pream housing. This can be relaced with a trip to Home Depot for a piece of 3/4 " x 3/4 " aluminum angle.

M

Hi Mat
LOL! Not much gets by you guys!
I started taking apart the amp before I took pictures.
The trim piece is alive and well and will get reinstalled directly.
Just wondering!
What would happen if an extension speaker cabinet was made with the displaced speakers?
Would a 4X12 system melt down the amp components?
It appears to be set up for extension speakers, but to what extent? Would more equal less?

RT

Hi RT: link to an eBay auction for a Jensen EM1200 similar to your speakers including pic of what's under the bell Here. Your amp's output transformer is designed for an 8 ohm load. Assuming you aren't playing Shea Stadium with it, you can safely operate it with as little as 4 ohms. If you have two internal speakers that are wired in parallel, the net load seen by the output transformer is already 4 ohms. If you connected two 8 ohm speakers wired in parallel to the spare / aux speaker out jack and that jack is wired in parallel with the internal speakers, then the load on the OT is reduced to 2 ohms.

Here is a link to Shavano Music that explains the impedance results for various wiring configurations. 4 - 8 ohm speakers in parallel - the internal speakers and a 2X? external cabinet represent 2 ohms of load. At high volume for extended periods, that's 2 ohms of resistance to a short. If you really wanna know if love is real and it has to be a 4X cabinet, most of the popular speaker makes are available with 16 ohm voice coils (4 - 16 ohm speakers in parallel = 4 ohm load) and, t'were me, I'd drill and install a DPDT switch to shunt the OT to the external cabinet; one or the other of the internal speakers or the external cabinet ... not both at the same time. Good luck! CJ

Hey Cap!
Yeah! Four 16 ohm speakers in parallel with an 8 ohm transformer handling 4 ohms of combined speaker power and cranked while sitting a mere 4 ft away in my 100 square feet of studio space.
Sounds like a good receipe for blowing up what I have left of my hearing, never mind the amp.
OK! I think I'm sort of getting it. Nuclear Physics was a tad confusing too.
So if you run four 8 ohm speakers in parallel down to 2 ohms, an 8 ohm transformer has a bad day. Thats interesting! Apparently ohms and amps don't follow the same logic such as running 10 amps on a 20 amp circuit and everybody is happy.
Thanks a lot! Glad I asked.
Moving on to Dark Matter and its influence on the Theory or Relativity and the possible existence of Worm Holes between Galaxies.
RT

RT 66 said:

So if you run four 8 ohm speakers in parallel down to 2 ohms, an 8 ohm transformer has a bad day. ... ohms and amps don't follow the same logic ... Moving on to Dark Matter and its influence on the Theory or Relativity and the possible existence of Worm Holes between Galaxies.

Click to expand...

Hi RT: The 'Half or Double' Rule applies to transformers; they can run at 1/2 or 2 times their design load .. that's it. If the load in ohms exceeds twice it's rating, it's pushing too hard. If there isn't enough resistance, the transformer can't keep up with demand.

Think of it this way; you go to a bar to 'get lucky' and all the chicks turn you down (high resistance); you react by being too aggressive, belligerent, drink too much and collapse in a drunken heap. Alternatively, you go to the bar and every chick decides to succumb to your bull (too little resistance); not too long thereafter, you're completely dissipated and useless ... but happy.

Yeah; those worm holes have been nagging at me too but right now, I'm trying to understand how electrical inductance divided by reactance equals inactive reluctance ... :wink:

Hey Cap
Finally an electrical analogy I can relate too. Its all so clear now.:mrgreen:
LOL! The Seeds video, LOL!
They look like the garage(garbage) band that played my high school dances.
I wonder if capes will make a come back?
I remember trench coat liners were considered cool for a while in my school until the ultra conservatives in control of dress codes and haircuts decided they weren't cool.
Good luck with your reluctant inductance problem.
RT

RT 66 said:

... The Seeds video ... I wonder if capes will make a come back ... I remember ... the ultra conservatives in control of dress codes and haircuts decided they weren't cool ...

Click to expand...

The Seeds ... the guy with the rug on his back ... the keyboard player dressed as Dracula ... and the woman go-go dancing in the foyer ... try and imagine the same scene with the Doors? At W&L HS in Arlington VA, they didn't want the kids wearing Levis to school ... so they told us the rivets used at the pocket corners would mar the desks, chairs, and seating ... Post those pics! :evil: :wink: CJ

I have nothing to add to this conversation but Babara Eden playing drums.

In regards to speaker load...a common way of using 4 -8 ohm speakers in to wire two sets into a series combination of 2-8 ohm speakers into 16 ohms and then connect each 16 ohm pair set in parallel with each other. the main thing is to be consistent with the wiring so that all of the speakers are in phase.

So basically you wire the speakers in the Thunderbird to 16 ohm series and connect it to the external 16 ohm series cabinet.

M

Thanks M; you're right, the series 8 ohm speakers would work too. CJ