Importance of Primary Impedance Output Transformer Matching?

[Bigjim63]

I have all of the TUT books but am still confused when it comes to matching Primary Impedance on Output Transformers to suit the particular tubes being used.  Furthermore, how important is it and what difference does it make?  

In regard to the construction projects in TUT 3 & 5 the most common specified output transformer for 50 watt power amps is a Hammond 1650KA.  This is being driven from a 4x octal tube complement.  The transformer primary impedance rating is 3,400 ohms with taps for 4,8,16 ohm speaker outputs.   You can find this transformer on the Hammond website under the general heading: Push Pull "Ultra Linear" HI-FI and it specifies for use with 4 Tubes (6L6GC, 807, 5881, EL34, 6146B, 6550B).  So far all good and it seems that this transformer is perfectly matched to a 4x Tube complement.

However, if you look further on the Hammond website and look at a direct Marshall JCM800 replacement output transformer for 50 watt you get the recommended 1750N. This has a primary impedance of 3,200 ohm.  But this is driving only 2x tubes!  Why is it so low?  Shouldn't it be higher at around 5000 to 6000 ohms if driving only 2 tubes? It is interesting to note that the 100watt JCM800 replacement transformer (Model No. 1750U) driving 4 tubes has a primary impedance of 1,700 ohms.  Do Marshall deliberately mismatch their primary impedances which gives them the sound they want?  How important is it to match impedances and what is the ideal impedance for a octal tube complement?  

If anyone can answer my questions and explain this confusion I'd be very grateful.

[K O'Connor]

Hi Guys

The fact is that there is NO specific match of a given tube type to a given OT primary impedance that one would assume and use universally; rather, as TUTs repeat, the range of OT impedances that all the common tube types can use is widely overlapped and that for the most common applications you can use ANY tube provided there is sufficient heater current and bias control range.

The TUT3 projects use _widely-available_ transformer types made by Hammond. At the time of writing, the 1750-series did not exist. That fact is actually immaterial inasmuch as the designs fall sort of into the "middle" of the operating range for the usual tube types: 6L6, EL-34, 6550, et al. The 1650K requires 292V peak at 343mA peak to achieve 100W peak output. With the usual 272JX PT, the unloaded Va is about 420V but will drop under load. Even if the supply stayed solidly at 420V, a single tube per side could easily drive the OT to 100W peak with only 25W peak dissipation over the audio cycle - which is about 18W continuous and well within the power ratings of all the tubes you mentioned.

Marshall used many surplus OTs and PTs in their production in the early years, so the "matching" is all over the map and supply regulation is not always stellar. In later years, cost still over-rode any other consideration.

Hammond's 1600-series OTs were designed for hifi applications but used in many guitar amps due to their good value-per-cost. As TUTs state, the bandwidth is narrower at higher than rated power levels, primarily as a loss of bass response and increase in distortion produced in the transformer itself. This is generally acceptable for MI, and thus you see the 1650T (120W rated, 1900aa) used in the Traynor Custom special at 160Wrms, and the 1650N (60W, 4k3aa) used in the 80W Bassmaster. (Traynor used simplified forms of the stock models). The OT in a 100W Twin reverb is about the equivalent of a 1600-series 45W model if one existed.

What OT primary you use depends on the power expected, the capability of the tube, and the voltage environment. There is no magic primary number for a given tube type. There is no single idle current for each tube type. You can use tubes to produce LESS power than their capability (most often the case). There is a lot more latitude in our way of thinking because we follow tube-safety-rules rather than tube-hifi-rules.

[Bigjim63]

Thanks Kevin,

What a great response and I appreciate the effort.  I'm sure this will be helpful to others as well because from reading other forums this subject seems to have many experts.

[K O'Connor]

Hi Guys

In the above examples of MI use of hifi OT designs, the extended power use is not great. The Fender PS-400 uses six 6550s to generate 450W continuous. The OT has three secondaries to match the 2x15" cabinets Fender designed for this amp - one amp + three cabinets as a standard rig. Modern players want to use their own modern cabinets which are usually 8-ohms or 4-ohms, but in any case, the player wants ALL the power available in one output. The Hammond 1650W is the exact same size and primary impedance as Fender's OT, but has  secondary that matches the player's needs. 1650W is rated for 280W, so here we have an extended use by a factor of 450/280=1.6 which is still small enough to allow massive low-end output.

Most people including "amp designers" tend to view the applications listed in the tube data sheets as "limiting cases" or "optimums", which leads to the idea that a given tube MUST work into a GIVEN primary load and can ONLY produce this much power. For many tubes, the plate curves are not available and/or the plots are at quite low screen voltages for tetrode/pentodes. We know from TUT2 and TUT4 how to scale these graphs to acquire full power from the tube and do it safely. If we read the data sheets carefully, we can see the real capabilities of the tubes we use and that there is much more overlap than difference. The important difference is their TONE.