Technical and Historical Frequently Asked Questions (FAQ)Last updated June 23, 2016
Q. What is the difference between the two-input and four-input HIWATTS?
A. Besides having two fewer input jacks, the two-input HIWATTS have a slightly different circuit with a bit more gain. They also use an ECC83/12AX7 in the PI position, which again provides a bit more gain. The older heads had an ECC81/12AT7 in the PI.
Q. What is a "Biacrown" HIWATT?
A. When Dave Reeves died in 1981, the Hylight company dissolved and some existing Hiwatt employees incorporated under the Biacrown name and continued to build Hiwatt amplifiers using the same (1981) materials and construction methods.
This new company did not do well financially, and had cash flow problems. This caused them to switch transformer vendors (away from Partridge), among other things. The major cosmetic difference was that they started to use rocker switches for power and standby in place of the original chrome toggles.
They also did a few minor circuit "experiments" to add more gain, to compete with popular tastes at the time (this is when the Marshall JCM800 2204 series were being made). This included an "OL" model, which just had more gain, and a "LEAD" model, which added a gain control labeled Overdrive in front of the OL gain mod (see further descriptions of these models below).
Some Hiwatt fans like these Biacrowns as much or better then the older models, because these models can produce the old original Hiwatt super clean sound, but also a slightly hairy overdrive, and all points in between.
The highest numbered Biacrown Hiwatt I have seen is S/N 16697. The latest Biacrown inspection sticker I have seen was dated 8/11/84 (S/N 16532).
Q. Which tube is the PI?
A. The small tube in the center of the three close together.
Q. Can I put an ECC83/12AX7 in the PI of my old Hiwatt (or an ECC81/12AT7 in the PI of my two-input Hiwatt)?
A. Sure - just replace the existing tube, no adjustment is necessary.
Q. Can I take two power tubes out of DR103 to get lower power?
A. Sure - remove the two outermost power tubes. You may want to halve the output impedance setting, as well to compensate for the change in output impedance cause by the loss of two tubes. For example, if the impedance selector was set to 16 ohms with four tubes, set it to 8 ohms when using two.
Output impedance matching is less important in this situation, because you'll only be putting half as much current through the OT. Mismatching the output impedance will cause even less power to be output, which may be a desirable thing!
Note that removing half of the output tubes this will approximately halve the output power, which is a change of -3db -- this will result in a similarly small change in volume -- a noticeable amount, but not "half as loud," as you might expect!
Q. Can I jumper the two input channels on a two-input Hiwatt?
A. Yes, but of course you'll need a "Y" cord or something similar. Both input channels are "in phase" and can be combined for different sounds.
Some people use an A/B/Y box for this, which also allows using one channel or the other as well.
Q. What year were the two-input Hiwatts first produced?
A. The two-input Hiwatts were first manufactured late in 1977.
Q. What's an "OL" model?
A. An "OL" is a Biacrown DR model with an extra gain stage added, which allows it to produce more preamp distortion than normal. These were produced during the Biacrown years, and were only designated "OL" on the serial number plate, e.g., "DR504OL" or sometimes "OL504."
Q. What's a "Lead" model?
A. A Biacrown DR model with an extra gain stage like the OL, but with an "OVERDRIVE" control, which was an added volume control before the extra gain stage. These were produced during the latter Biacrown years, and were labeled on the front panel, e.g. "HIWATT LEAD 100" or "HIWATT LEAD 50".
Q. What's a "DR105?"
A. This is a DR103 modified specifically for the Canadian Market, in order to meet stricter CSA electrical requirements. The main difference is that it had no voltage selector and came hard-wired for 115 VAC mains. Some later models also had extra safety fuses added inside. Note that coincidentally the Canadian distributor of Hiwatt products at the time also added a "mod" to the pre-amp section which provided an odd sort of preamp gain, something like a Marshall JCM800.
Q. What's a "CP103?"
A. This is a later version of the amps that Dave Reeves built on contract for Sound City during the early years of the Hylight company. These were produced on a custom basis for Pete Townshend of The Who. It uses the DR103 power supply and output section, but has four separate inputs, first stage preamps and volume controls, two of which were voiced brighter. Is also has a very different tone control circuit with no middle or presence controls. In all the pictures of Pete's rigs I've seen, only one input is being used. The later front panels of CP103s built for Pete Townshend read CUSTOM HIWATT 100 "The Who." Some pictures are available here.
Q. What's a "DG103?"
A. This is a slightly modified version of the classic DR103 amps, built on a custom basis for David Gilmour of Pink Floyd fame. These amps have a brilliant input, a normal input, and a "bridged" input, which drives both channels. The fourth input hole (upper left) is usually plugged. It is not clear if these came from the factory with inputs wired this way, or if they were so modified by Pete Cornish, who has maintained them since they were new.
Q. What's a "FL112", "CL112?" and "AL112?"
A. In the 1979-1981 timeframe, these designations were used on combos to indicate the type of speaker inside:
Occasionally a model came out with a variation on this naming scheme, e.g., "SA112FL."
Q. I heard that the Biacrown HIWATTs didn't use Partridge transformers. Doesn't this change the sound?
A. Particularly near the end in 1983-1984 other transformer manufacturers (Drake and Sowter) were used. We think these Biacrowns all sound great, and just as "HIWATT-like" as the older models.
Note that Harry Joyce used Drake transformers in the highly regarded Hiwatt-like amps he produced in the early 90s.
Q. Are the point-to-point models better than the ones with printed circuit boards?
A. First of all, HIWATTs were never made point-to-point, but rather used tag boards with turrets. See this page for a description of the differences. Dave Reeves started using PC boards in his Hiwatt amps around 1979. At the same time, all the filter cap cans were moved to the inside of the chassis.
In our opinion, there is no difference in sound or reliability between the PC board and turret board models (given identical circuits, of course).
Q. Which tubes should I use to re-tube my Hiwatt.
A. The best ones you can afford. :-) Ask ten people and get eight different answers. The OEM tubes installed at the factory were made by the original Mullard or Gold Lion companies. Unfortunately, the tubes being manufactured in Russia using these names now are not of the same quality or specification. The original tubes have gotten incredibly expensive and difficult to find on the NOS market. For power tubes we like the EL34s sourced and selected by TAD. For preamp tubes, we still like NOS best, but the TAD HIGRADE 12AX7s and JJ ECC83S seem to work well.
Q. My Hiwatt is (30..40) years old. Should I get it re-capped?
A. If you're using it regularly to play out, yes. Electrolytic caps are made using an electrolytic paste that will dry out over time. At best, the caps are not working as well as they should, which causes increased hum and decreased punch and dynamic range. At worst they can fail in a spectacular and messy fashion. This failure can happen at the most inopportune times, and possibly cause other damage when it does. Thirty or more years is a very long time in service for an electrolytic cap.
Q. What is the "Slave" jack on the back of my Hiwatt?
A. The "Slave" jack on the back is an output, meant to drive one or more Hiwatt Slave amps. The Slaves were basically amp heads without most of the preamp circuit and no tone controls. The idea was you could use two or more amps/stacks but only have to adjust one set of controls.
Note that this output is derived directly from the speaker output, so it contains the same exact signal that the loudspeakers get.
Q. Why do some Hiwatts have very small serial plates?
A. According to Ian Pogson (a former Hiwatt Park Road employee), "There is a simple answer to the cut down serial plate: From time to time we ran out of blank (unstamped) plates and had to cut down a plate used for loudspeaker cabinets! ...a tedious job was stamping out these serial plates with consecutive numbers, model type was added at the time of fixing to the chassis and noted in a loose leaf pad!"
Q. Why do some Hiwatts have "T/S" written in marker pen on the inside of the chassis?
A. Again, from Ian Pogson: " A genuine Park Road amplifier will have 'T/S' in marker pen written on the left hand side of the chassis, [meaning] 'Tested' and 'Soaked'."
Q. What is the thread size of the screws that fix the chassis to the head cabinet?
A. The original Hylight machine screws were British Imperial #2 (2BA) thread. These can be hard to find now, particularly outside of the U.K.
The metric M5 x .08 and US 10-32 threads are close to 2BA. In either of these latter cases, We'd recommend getting a tap of the appropriate size and running it through the chassis nuts so the that the replacement screws do not bind. The tap barely touches the threads, so it's very easy to do.
Q. What's the correct way to connect speakers to the four output jacks on the DR201 and DR405?
A. This is a simple question that does not have a simple answer!
Some of the earliest DR201 models simply had all four output jacks connected in parallel, so there's no mystery.
Next, DR201 models with KT88s had two pairs of DR103-style jack connections in parallel. The intention of this connection was to enforce the use of at least two cabinets. In this case, you need to plug one speaker into one of the two leftmost jacks, and another speaker into one of the two rightmost jacks.
If you want to use one high-power cabinet (and get around this issue), you need to insert an unattached 1/4" plug into the pair of output jacks you're not using to open the shorting jack connection.
The later (6xEL34) 201s and the 405s used a more complicated switching setup. The four jacks are separated into two pairs, the two closest to the selector and the two further away. These two pairs of jacks are internally connected in series with each other. This provides a dizzying array of hookup options, and will allow for up to sixteen(!) normal Hiwatt 16 ohm cabinets to be safely connected. We'll show just one or two of the many hookup options below for each "cabinet collection."
Note that the only numbers of cabinets that will provide a balanced impedance are one, two, four, eight, or sixteen. Three cabs can be connected, but the impedance (and distribution of output power) will be somewhat mismatched.
I've arbitrarily numbered the jacks 1 to 4, with 1 being the jack closet to the impedance selector.
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