OT questions regarding DCR

[chas.wahl]

I'm working on a clone of an amp design (Ampeg R-12-R Reverberocket) that has a push-pull OT specified as 10k Ω primary (300 Ω DCR), 8 Ω secondary (0.7 Ω DCR). It's a 2 x 6V6 amp with a paraphase PI, so the OT was probably rated at 15 W or so.

I bought a ClassicTone OT (40-18037 intended as repl. for Marshall 18 W) with similar impedance specs: 9.2k Ω primary and multiple outputs at 4, 8, 16 Ω. It's a fairly substantial one weighing 2#-3oz or 0.98 kg with mount spacing 3-9/16" -- a slightly bigger stance than a Fender 125A1A, for instance.

Now here's my concern: the DCR measured for the primary on the ClassicTone is 640 Ω end to end of the primary (310 and 330 per side) -- over twice what the spec by Ampeg is. The secondary DCRs for the 3 taps are all in the same order of magnitude as suggested by Ampeg, 0.4 to 0.7 Ω (though measuring single-digit ohms is very unreliable given the multimeter I have).
EDIT: in case anyone was going to ask -- No, I don't know what the spec or actual inductance is for either of these, nor their turns ratios. I suppose with a low-voltage setup I could establish the latter for the one I have in hand; but I haven't yet.

If I compare the specs of various similar Hammond offerings having roughly the same primary impedance, I find the following:
1760E (15 W, repl. for Fender 5E3 Deluxe, 8.5k primary): about 310 Ω end-to-end
1760H (20 W, repl. for Fender AB763 Deluxe Reverb, 6.6k primary): about 350 Ω end-to-end
1750PA (18 W, repl. for Marshall 18 W, 8.4k primary): about 635 Ω end-to-end
So, it doesn't seem to be a fluke that the Marshall OT does have a higher primary DCR than comparable Fender OTs.

Is that difference in primary DCR spec a problem? If the Ampeg spec is indeed end-to-end and the two OTs really have a double/half relationship in their primary DCRs, then what would that difference result in, in terms of the change in OT performance?

Should I be looking for an OT with 8.5k to 10k primary impedance and a lower primary DCR? I imagine I could sell the (now unobtanium) ClassicTone for what I paid easily enough. But I haven't found a better match, except for the "original" replacement OT-149 (by Heyboer) from Fliptops.net, who don't seem to be very forthcoming about specs, and have a pretty steep price compared to the ClassicTone.

 

[chas.wahl]

OK, either
1) I'm being shunned (entirely possible, and I'm just too insensitive to recognize it) or else
2) TDPRI community just doesn't care about anything Marshall-ish, and so has no relevant experience, or
3) My question is too complicated.

In order to simplify the question, I'll try again:
Fender amp OTs for medium-wattage push-pull types (along with the Ampeg Reverberocket I'm interested in) have a primary DCR end-to-end of (ballpark) 300 Ω or so.
Marshall amps of similar wattage, for instance the 18 watt, have an OT with primary about twice that DCR.
What might be the reason for that?
And could I use a Marshall-type OT in an amp whose design spec for OT was more along the lines of the Fender?
And if I could, what difference might that make to the original sound?

It has occurred to me to try this on at EL34world, where there are a lot of Marshall lovers -- but this is the place I typically go first.

 

[peteb]

In order to simplify the question, I'll try again:
Fender amp OTs for medium-wattage push-pull types (along with the Ampeg Reverberocket I'm interested in) have a primary DCR end-to-end of (ballpark) 300 Ω or so.
Marshall amps of similar wattage, for instance the 18 watt, have an OT with primary about twice that DCR.
What might be the reason for that?

I am not able to answer your question.

it is not the DCR that matters. For instance, a 50 W Bassman measures 45-45 DCR. Where is the logic in that?
different wire gauge?

It is not the DCR that matters. I suggest backing up.


what is the speaker load?

what is the winds or turns ratio of the OT?

the turns ratio squared, is the impedance ratio of the OT. It is a multiplier that multiplies the speaker load into the total load.


square the turns ratio and multiply it by the speaker load.

this is the impedance load the power tubes are working into.

you could do this calc for Marshall and fender and then compare.

 

[andrewRneumann]

I suspect the one with the higher resistance uses more turns of wire on the primary and the secondary. This may result in the same impedance ratio, but more inductance. The upshot is that the OT with more turns of wire has more inductance, and better low frequency response. It's also bigger, heavier, costs more, and drops more B+ at the power tube plates.

Do you have any frequency response graphs for the two OTs?

To your original question--if you are building a clone, you want an OT that has the same primary impedance and primary inductance as the original. If you don't know the inductance, then the best second option is to do what you are doing--buy an OT with similar power rating and similar primary DCR and hope for the best. In the end, either OT will work, but the low frequency response will be different.

 

[chas.wahl]

@peteb:
I know that the Marshall 18 Watt OT has the correct impedance for the Ampeg amp (ca. 10k) when used with 8 ohm speaker load, and also know the primary impedance varies a bit when 4- or 16-Ω taps on the secondary are used. I don't think that's a problem. I also know that, generally, for higher-output amps with bigger OTs, the DCR of the primary is typically lower, sometimes quite a bit lower, than for OTs in the 15–18 watt category. "The logic in that" is that the bigger OTs have lower primary impedances typically, and use bigger gauge (less resistive) wire to achieve that impedance in the primary winding.

It's just nagging me that there must be a reason why Marshall's 18 W OT has a DCR twice that of a comparable Fender OT, and I'd like to understand what that reason is, and how it might affect things. As I understand things, either both windings in the Marshall have more turns (and greater wire length, thus more resistance) but comparable turns ratio; or else the Marshall OT is wound with smaller gauge wire (more resistance again), even though it's a pretty hefty OT -- which might be a concern, and affect how the OT reacts in use. I don't have a meter that will tell me with any accuracy the resistance of the secondary winding.

As I said in my first post, I could buy a "replacement" OT that's supposed to have the original specs, but a) Fliptops is not very forthcoming with spec data, simply touting it as a replacement, yet b) the Reverberocket design went through a lot of changes through the years, such as replacing all the octal preamp tubes with novals, changing the output tubes for higher-watt ones (maybe more than once), getting rid of the tube rectifier, so when somebody says "replacement for Reverberocket" I'm not sure that I trust they used the same OT to cover all those bases, and didn't monkey with it to make all these (unwanted, in my case) "improvements" as well.

At least I know the specs for the Marshall replacement OT, and except for the primary DCR, they're spot on for the particular circuit I want to build.

I suspect the one with the higher resistance uses more turns of wire on the primary and the secondary. This may result in the same impedance ratio, but more inductance. The upshot is that the OT with more turns of wire has more inductance, and better low frequency response. It's also bigger, heavier, costs more, and drops more B+ at the power tube plates.

OK, that was one of the possibilities I mentioned just above, and maybe that's OK. Since the PT I'm using is "vintage" with 117 V primary rating, but the same output V, having a bit of drag on the PT output might work well enough (though a concern is that both the original PT and the one I have are rated 90 mA for HT winding, and I calculate the actual mA required for all the tubes as somewhat higher than that -- can we say "sag"? -- but also heat?). Just another thing to worry about.

Do you have any frequency response graphs for the two OTs?

No. For neither; next to nothing for the Ampeg (primary and secondary impedances and DCR in Ampeg service manual, but only "replacement for" from Fliptops, no specs at all) and no graphs nor inductance from ClassicTone (RIP), though pretty good documentation of the impedances otherwise.

To your original question--if you are building a clone, you want an OT that has the same primary impedance and primary inductance as the original. If you don't know the inductance, then the best second option is to do what you are doing--buy an OT with similar power rating and similar primary DCR and hope for the best. In the end, either OT will work, but the low frequency response will be different.

OK, thanks. My feeling at this point is that, rather than pay $150 for the Fliptops replacement, I'll try out the Marshall 18W replacement and see how that goes.

The EL34world denizen Sonny Reverb who built a clone (the only one I'm aware of) used ClassicTone 40-18022 OT, which is (was) the basic Tweed Deluxe one with 8k rather than 10k primary impedance. I thought the Marshall 18W one would be "closer". There's also Hammond's 1760E, with an 8.5k primary. That would be an easy alternate if it comes to it.

Thank you both for your help,

 

[andrewRneumann]

The other possibility is that the inductance is the same or similar but that the wire gage is just smaller to allow room for more secondary windings (16Ω requires about 40% more turns than 8Ω if I did my math right).

I compared the Hammond Fender Deluxe Replacements and the Marshall Replacements and the Marshall has a lot more inductance. Interestingly, both have enough inductance to handle the low frequencies just fine. So there may not be a large difference in the low end.

Something I learned was that the extra windings in the Marshall OT actually cause more loss of really high audio frequencies. I believe this is because the extra windings lead to more inter-winding capacitance. You might find the Marshall OT loses a little of the brilliance above 10kHz compared to Fender replacements. (I'm assuming the Ampeg replacement is similar to a Fender replacement.)

 

[bebopbrain]

Is that difference in primary DCR spec a problem?

No. If the impedance is 10K then any voltage drop due to 300 ohms is no big deal.

Use the big one. It is probably is wound with the same wire but with more turns for more inductance. Counterintuitively, more resistance may be better. As @andrewRneumann said, the bass response should be an improvement.

 

[chas.wahl]

Something I learned was that the extra windings in the Marshall OT actually cause more loss of really high audio frequencies. I believe this is because the extra windings lead to more inter-winding capacitance. You might find the Marshall OT loses a little of the brilliance above 10kHz compared to Fender replacements. (I'm assuming the Ampeg replacement is similar to a Fender replacement.)

Well, my other project in planning is a Tweed Deluxe head (quite a bit simpler) and that will use the Hammond 1760E (8.5k to 4|8|16 Ω) unless I can find one of the ClassicTone versions at 8k with 3 secondary taps. So I'll have that transformer to pop into the Reverberocket, and check out the difference. Things to watch out for -- thanks!

 

[2L man]

Chas, more wounds increase the inductance which produce less power loss on low frequencys. H1750PA power line has not drop much at 50Hz.

Edit: H1760E and H1760H has not drop at 50Hz neither so 1750PA higher inductance has not much effect to guitar use.

2nd edit: I don't do well now as I just saw the picture of your PT on other thead and on its label says 90mA DC so first half of the following paragraph does not cope now Obviously there is another way to rate transformers? European way is to rate AC current and/or VA (volt amps).

If transsformer is rated for 90mAAC and it has a CT and is full wave rectified using two diodes and filtered, then its filtered DC current capacity incease by 1,41. This because secondary halfs alternate and although higher current heat the coil doubled it can cool when other coil transfers the current. So about 125mA filtered DC is the RMS current it can deliver. In guitar use the peak current often can come quite a lot higher when playing has loud dynamics but containing also low volume sections.

 

[Lowerleftcoast]

I built a 5E3 with the Stancor PM-8409 PT and a Hammond 1750PA OT.

Idk if yours will sound exactly like the Ampeg... but the transformers made a nice sound in my build.

I think this choice will be very close to the Ampeg. I doubt one would hear a difference using a lower DCR output transformer all else being equal. I bet that would be a very close listening test, A vs B.

 

[chas.wahl]

I think that what I'm taking away from this is that the ClassicTone 40-18037 that I have should work fine impedance-wise, may have more inductance than the OT-149 that was used in the Reverberocket 1962 model (better bass support, maybe attenuate some highs or be "less brilliant"), may drop more voltage than expected. Also that an OT like the Hammond 1760E may work fine, a bit less primary impedance than the Ampeg had, so maybe cleaner, but probably similar inductance to the OT-149.

Thank you all.

 

[peteb]

Here is what I see on the Hammond specs

fender 15 W: P-T1750E, p-T1750EP

speaker load: 8 ohm
turns ratio: ?
impedance ratio: ?
Impedance: 8.5K

speaker load: 8 ohms
turns ratio: square root of 1,062.5 is 32.6
impedance ratio: 8.5k / 8 = 1,062.5
Impedance: 8.5K ohms


Marshall 18W: P-T1750PA

speaker load: 8 ohms
turns ratio: ?
impedance ratio: ?
Impedance: 8.4K ohms

speaker load: 8 ohms
turns ratio: square root of 1,050 is 32.4
impedance ratio: 8.4K / 8 = 1,050
Impedance: 8.4K ohms



as far as those specs go, they look very similar.


it is interesting that DCR of the primary is an important parameter for maintaining an amp, and good to know as LLC wrote, but it is an unpublished number as far as I am concerned.

 

[andrewRneumann]

I think that what I'm taking away from this is that the ClassicTone 40-18037 that I have should work fine impedance-wise, may have more inductance than the OT-149 that was used in the Reverberocket 1962 model (better bass support, maybe attenuate some highs or be "less brilliant"), may drop more voltage than expected. Also that an OT like the Hammond 1760E may work fine, a bit less primary impedance than the Ampeg had, so maybe cleaner, but probably similar inductance to the OT-149.

Thank you all.

I also want to make another comment about reading the frequency response charts that Hammond gives. When I said that both OT’s can handle the low end just fine, I didn’t take into account the Rs (source resistance) of the output valves. The chart assumes impedance matching… the 8.5k winding is matched with an Rs of 8.5k for maximum power transfer. The source resistance of beam tetrodes is actually quite a bit higher than that so that is going to change the frequency response. To sum up—I actually don’t know if both of those OTs have similar bass response at guitar frequencies when paired with 6V6s. It should be modeled to find out. Peace out my bruthas.

 

[chas.wahl]

The (Marshall 18 W replacement) I have for this project does have a somewhat higher primary impedance (for 8 Ω tap) that's clearly stated on the ClassicTone literature:

 

[peteb]

Hammond is really thorough,

they show the DCR on the primary and secondary.

I have never heard of anyone referring to these values.

15 W fender

https://www.hammfg.com/files/parts/pdf/1750E.pdf

18 W marshall

https://www.hammfg.com/files/parts/pdf/1750PA.pdf

The (Marshall 18 W replacement) I have for this project does have a somewhat higher primary impedance (for 8 Ω tap) that's clearly stated on the ClassicTone literature:

even more so, I believe the difference is that the primary and secondary have more DCR.

the Hammond 18 W, which does specify primary DCR around 300-300, also has a higher secondary DCR, 1 ohm, I calculate. (If anyone would like to see the nifty calculation, I can share)

As expected, to maintain turns ratio, the DCR on the primaries and secondaries are about double on the 18 W compared to the 15 W.


to increase DCR, either number of winds is increased or the gauge becomes smaller, physically.


the 18 W is physically larger than the 15 W.

it appears the 18 W has increased windings, maybe double the 15W.

in my estimation, doubling the winds does not add power, but probably, more bandwidth.