Convert an 18Watt to a 36 Watt

dustin_writes · Post by **dustin_writes** » Fri 10/23/09 7:22 am

Hi,
I'm wondering if someone could give a general comment about converting an 18 watt to a 36 watt.

Here's how I see it:
-Bigger Transformers
-two additional el84s or two 6l6 (not a purist option, I realize)
connect the whole sh*ttery as per schematics.

Comments?

Thanks!

Post by **rjgtr** » Fri 10/23/09 7:25 am

If you use the GDS 36watt transformer set you can. They fit the same holes as the normal 18watt transformers. Other than changing the transformers you'll need 2-EL84s, grid resistors for the additional tubes and a new cathode resistor for the power tubes. That's it, the rest can stay the same.

dustin_writes · Post by **dustin_writes** » Fri 10/23/09 7:40 am

You are like a god to me!

Phil_S · Post by **Phil_S** » Fri 10/23/09 7:55 am

Does your chassis have room for 2 more noval sockets? By this, I mean you've got to consider what's on the inside. Just because you've got space on the outside doesn't mean it will work from the inside.

Alternatively, if you are converting to a pair of octal sockets, are the existing noval socket holes far enough apart to accommodate this? Not all octal tubes are equal. For example, a pair of KT66 (big bottles) requires more lateral spacing than a pair of comparatively svelt 6L6 or EL34.

dustin_writes · Post by **dustin_writes** » Fri 10/23/09 9:58 am

I believe I have enough room for octal sockets [correction, sorry about that] . Is that more a question of depth as slighty bigger holes shouldn't be that big of a deal.

After working on a '66 Bandmaster I don't recall those to be especially large. I would be using 6L6s, if for no other reason, out of familiarity.

Question: the 2x6l6 build will require yet another transformer set vs the 4x el84 design, correct?

Post by **CurtissRobin** » Fri 10/23/09 11:48 am

dustin_writes wrote:I believe I have enough room for nuval sockets. Is that more a question of depth as slighty bigger holes shouldn't be that big of a deal.

After working on a '66 Bandmaster I don't recall those to be especially large. I would be using 6L6s, if for no other reason, out of familiarity.

Question: the 2x6l6 build will require yet another transformer set vs the 4x el84 design, correct?

"Noval" means nine-pin, the EL84 sockets. You need to have room for two more sockets (and related parts) if you're going to use 4xEL84. If you change from two EL84 to two 6L6 then you'll need to enlarge the holes for octal sockets. You'll also need to see to it that there's enough room for circulation of cooling air around the tubes since 6L6s are much larger (diameter, not height) than EL84s. Finally, if you go to 6L6 be prepared for a total change in the amp's sound. Even if it was originally built as a genuine Marshall 1973 it'll scarcely sound like an 18 watt amp which gets a huge portion of its distinctive sound from the slamming of the EL84 power tubes.

Yes, a different transformer will be needed to change to 6L6 power tubes. Pull out the data sheets for both power tube types and compare them.

KennyO

Shrapnel · Post by **Shrapnel** » Fri 10/23/09 1:31 pm

Could use 7591s... 19W max plate diss. can squeeze 45w out of as pair (450v) 37w (400v) 30w (350v). Similar in that respect to a 6L6GB... BUT gain structure like an EL84 (Use the same 18Watt EL84 PI and PI component values to drive 'em.) morcey2 has used them before in a plexi7591 type build.

WARNING: old-stock comes at a premium as the Hi-Fi nutz are ga-ga over the 7591 JJ & EH make new versions.

krx · Post by **krx** » Fri 10/23/09 2:58 pm

Shrapnel wrote:Could use 7591s...

For that great Ampeg overdrive?

dustin_writes · Post by **dustin_writes** » Fri 10/23/09 3:04 pm

So 7591would give a close match to the el84 tone, require only two tubes, but would still have an enlarged diameter that may create too much heat/ limited ventilation. Right?

Shrapnel · Post by **Shrapnel** » Sat 10/24/09 1:49 am

Bottle size is close to a 6V6. Pinout is VERY different. But you can get the power of 4xEL84 or 4x6V6 out of two. The 7591 has the gain structure of the EL84, but I can't personally say that it matches the sound of the EL84... at least yet. (Working on a project utilizing 2x7591, but can't go too much into detail here... the PI however is a slightly modified value 18Watt PI, but has NFB on the second input into the PI (Very un-18Watt like) The rest is non-18Watt material (2ch- 1.Marshall Plexi 2.Soldano SLO.)

The 7591, besides finding its way into organs and Hi-Fi, were used in some Ampegs and Gibson amps... but look to be viable 36Watt contenders w/ less mods than for 6V6s (other than obvious screen and
biasing considerations.)

Phil_S · Post by **Phil_S** » Sat 10/24/09 9:08 am

dustin_writes wrote:So 7591would ... require only two tubes, but would still have an enlarged diameter that may create too much heat/ limited ventilation. Right?

You only need about 1/2" between the bottles. More would be better.

An octal base is about 1 1/4" and a noval base is about 7/8" If you have a 1" gap between the two EL84's, and assuming you use a step bit to enlarge the holes, you'll lose 3/8" or so. It's workable from that standpoint. I have no idea what you might be up against inside the amp.

krx · Post by **krx** » Sat 10/24/09 4:43 pm

dustin_writes wrote:So 7591would give a close match to the el84 tone, require only two tubes, but would still have an enlarged diameter that may create too much heat/ limited ventilation. Right?

The tone will definitely not be the same. They just have a similar amount of gain so you don't have to change the PI. I've personally never heard a 7591 guitar amp that sounded good overdriven.

Shrapnel · Post by **Shrapnel** » Sat 10/24/09 10:19 pm

From PPWatt.com's Plexi6v6 section

morcey2 wrote: Compared to the 6V6 build, the power tube breakup is more aggressive, similar to EL34s. I don't have anything to compare directly with that uses EL34s, but it sounds like the one's I have heard in the past. I'm using JJ 7591's, reissue tung-sol 12ax7's, and chinese 5AR4. All the signal caps are M150's and silver-mica. The resistors are a mix of 1/2W metal film and carbon film, with the exception of the PI plate loads which are carbon comp.

Matt.

morcey2 wrote:
Shrapnel wrote:If so, Would you be willing to share your schem of the PA section of your Plexi7591? And how satisfied you are/were with the final build's sound?
I'm still here. I really like the sound but I haven't really had lots of time with the amp. The PA is basically the same as the Plexi6V6, but with the octal sockets wired for 7591's. I'm eventually going to change the tail resistor on the PI from 10k to something more like 33k or 47k because the 7591's have gain similar to EL84's. I think I used 100k grid leaks and 5.1k grid stoppers. I'm not sure what I used on the screens, but I think it was 1k each. This one doesn't have a choke, but I'll probably add one next time put in an order. It's just not quite as tight as I would like it to be and I think that's the missing choke doing that.

Matt

Ok, now before this post can get nuked... these quotes are referenced for the sound of the 7591s. Morcey2 likes the sound of the 7591s, krx apparently doesn't, at least in the circuits he's heard. YMMV as to where your ears fall in liking them.

Just note that the 7591s like 6k6-9k trannies, are wired different than a 6V6, 6L6, and EL34. Rewire the sockets and adjust the PI resistors if you try this and don't like the 7591s. I still think they may be good candidates for a 36 Watter... Just depends on how much you feel like experimenting.

Cknopp · Post by **Cknopp** » Sat 11/07/09 9:57 am

For a newb here,

What, exactly, is the difference between an 18watt and 36watt power transformer? Is it the secondary voltage (b+), the mA, or is it completely different?

Thanks! I appreciate the education that is so freely given here!

Thank you!

Chris

Shrapnel · Post by **Shrapnel** » Thu 11/12/09 11:19 pm

I think what you'll find is the voltage will remain the same, but the available current delivery will have increased. If using a Tube Recto, that will probably change as well to something like the GZ34. (also means a 5v winding)

Snooty · Post by **Snooty** » Fri 11/20/09 9:42 pm

Instead of say 150mA you will need 200 or 300mA for 36watter.

Mark

V_amp · Post by **V_amp** » Sat 01/23/10 8:19 pm

Hello, I have the same wish that dustin_writes who started this topic.

I will use 4 el84.

So some characteristics of my PT and OT Transformers:

PT: 300Vac CT 130mA / 6,3Vca CT 3,5A and 6,3VAC 1A for EZ81.

OT: primary impedance 8kOHms/ 110mA / secundary impedance 4 and 8 Ohm.

I imagine I need a 4k primary impedance for OT , but can I start with I have in my hands?

Can I use these trasnformers to start my experiences?

____NEXT STEPS__________________

My ampli is by dockery (attached)
I saw a second schematic by Richie and Mark rev-5 (attached too) an it encourage-me!!!!

My ideas are:
1) Return SS rectification Using SAG resistor of 220-OHM 10W.

2) don't wire a new 150-Ohm bias resistor like in Richie and Mark schematic, only replace the 150-Ohm bias resistor for 75-Ohm (because I need two in parallel for 36W ! ? So, I will wire all pin-3 from el84 at the same point. Ok?

3)don't wire a new 470uF, only replace the 470uF capacitor for 1000uF, because I need two in parallel !? (the same reason: all pin 3 connected at the same point) Ok?

4) don't wire a new ones 8.2k resistor, only pelace them for more watter like 8.2k 3W watts, and wire from them two wires directly t the pin 2 two the tubes tubes. (4 wires, one for each tube, two wires for each resistor of 8.2k) Ok?

5) replace the first two capacitors of 10nF of the power push-pull stage to two of 22nF as in the Richie and Mark schematic.

Opinions? What the gurus think about.
See, I'm starting the mods, in the future I will replace the transformers.

Can I heat my iron solder ?

Thanks all!

FAB

V_amp · Post by **V_amp** » Sat 01/23/10 9:03 pm

Ok, I'm convinced by myself that answer for the question 2 and 3 is NOT! An it is easy to wire.

...And the 8.2k are too easy to wire.

About the PT, it will be hot fast.

But the last one....About the OT? Can I try the same?

Thanks
FAB

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