
384 Mass. 390 (1981)
425 N.E.2d 294
COMMONWEALTH
vs.
A JUVENILE (No. 2).
Supreme Judicial Court of Massachusetts, Plymouth.
May 7, 1981.
August 19, 1981.
Present: HENNESSEY, C.J., WILKINS, ABRAMS, NOLAN, & LYNCH, JJ.
Joseph F. Killion for the defendant.
Rosemary Ford, Assistant District Attorney, for the Commonwealth.
WILKINS, J.
The defendant juvenile was charged in a District Court complaint with delinquency by reason of committing, on January 10, 1980, assault and battery by means of a dangerous weapon, in violation of G.L.c. 265, § 15A. After an adjudication of delinquency by a judge of the District Court, the defendant appealed to a jury session *391 in the District Court under G.L.c. 119, § 56. Before empanelment of the jury, the defendant moved for a trial to a jury of twelve persons. The judge denied the motion and, on August 15, 1980, he was found delinquent by a jury of six. On appeal, he challenges the denial of his motion for trial to a twelve-person jury. He also argues that he was improperly denied the right to impeach the victim's credibility by reference to the victim's juvenile record. We transferred the appeal to this court on our own motion, and we now reverse on the ground that the defendant was entitled to a trial by a jury of twelve.
1. On appeal to a jury session of a Juvenile Court from an adjudication of delinquency, a juvenile is entitled to a jury of twelve "in those cases where trial would be only upon an indictment were the child an adult." G.L.c. 119, § 56, as amended through St. 1979, c. 344, § 1. The Commonwealth grants that at the time of the alleged delinquency, and at the time of the filing of the complaint, a juvenile was entitled to claim a trial before a jury of twelve. The crime charged is punishable by a State prison sentence of more than five years (see G.L.c. 265, § 15A), and, when the crime was committed and the complaint was filed, the crime was not within the jurisdiction of the District Court Department (see G.L.c. 218, § 26, as amended through St. 1978, c. 478, § 187). If the juvenile had been an adult, therefore, he could have been tried only on an indictment, unless he waived indictment. See G.L.c. 263, § 4; Mass. R. Crim. P. 3 (b), 378 Mass. 847 (1979).[1]
*392 The Commonwealth argues that the defendant waived his right to a trial before a twelve-person jury. The record before us concerning the filing of the defendant's motion for a jury of twelve is sketchy. According to the parties' "agreed statement of facts," the defendant filed a motion on August 14, 1980, prior to empanelment of the jury, claiming trial before a jury of twelve pursuant to G.L.c. 119, § 56. The motion was denied, and the jury returned a verdict of delinquency on the next day.
We are unable to determine that the motion was not filed in a timely fashion. The judge had discretion to allow the motion to be filed at the time it was filed. See Mass. R. Crim. P. 13 (d) (2) (A), 378 Mass. 873 (1979). The action of the judge in hearing the motion imports a preliminary determination to allow the motion to be filed. We accept, therefore, that the defendant's motion was filed seasonably.
The Commonwealth next contends that, even if the defendant was deprived of his statutory right to a jury of twelve, that error did not prejudice him and, therefore, reversal of the adjudication of delinquency is not required. Ordinarily, statutory violations are not grounds for reversal unless the violation caused some harm. See Commonwealth v. Hartford, 346 Mass. 482, 488-489 (1963); Commonwealth v. Pike, 324 Mass. 335, 337-338 (1949). The *393 burden is on the defendant to show prejudice. Commonwealth v. DeChristoforo, 360 Mass. 531, 536 n. 2 (1971). But, where actual prejudice is impossible to establish and where the right denied is substantial, we have reversed convictions without requiring a defendant to show actual prejudice. See Commonwealth v. Tabor, 376 Mass. 811, 819 & n. 13 (1978); Commonwealth v. Johnson, 365 Mass. 534, 547 (1974), and cases cited. Cf. Searle v. Roman Catholic Bishop, 203 Mass. 493, 499-501 (1909). As to minor matters where prejudice could not be shown, the result has been different. See Commonwealth v. Montecalvo, 367 Mass. 46, 51 (1975) (interrogation of jurors not conducted under oath); Commonwealth v. Guerro, 357 Mass. 741, 755-756 (1970) (clerk rather than judge asked jurors the statutorily prescribed questions).
The question then is whether the statutory right to a twelve-person jury is sufficiently substantial so as to require protection of the right, even in the absence of any demonstrable prejudice to the defendant. Certainly, no defendant could prove that he was prejudiced by the reduction of the size of the jury, any more than the Commonwealth could prove that he was not. It may be true that the defendant did not have a constitutional right to a jury of twelve. See Opinions of the Justices, 360 Mass. 877, 881-885 (1971); Williams v. Florida, 399 U.S. 78, 86-103 (1970). But see Opinions of the Justices, supra at 886-887 (Quirico, J., dissenting), where Justice Quirico expressed the opinion that a reduction in the size of a jury below twelve was so substantial that it could be achieved only by a change in the Constitution of the Commonwealth. However, a jury of less than six persons would deprive a defendant of his right to a jury trial guaranteed to him by the Sixth and Fourteenth Amendments to the Constitution of the United States. Balew v. Georgia, 435 U.S. 223 (1978). In other States, where the right to a trial by a jury of twelve is guaranteed by Constitutions, deprivation of that right has been held to constitute reversible error without a showing of prejudice. See State v. Miguel, 125 Ariz. 538, 540-541 (1980); Brame v. *394 Garwood, 339 So.2d 978, 979 (Miss. 1976); Hartgraves v. Don Cartage Co., 63 Ill.2d 425, 427-428 (1976); State v. James, 30 Utah 2d 32, 36 (1973). Reversal has similarly been required where the right to a twelve-person jury is solely statutory. See Bourne v. Atchinson, Topeka & Santa Fe Ry., 209 Kan. 511, 516-519 (1972); State ex rel. Polk v. Johnson, 47 Wis.2d 207, 214 (1970).
The statutory right to a jury of twelve was a substantial right, and that right should have been protected, even in the absence of any demonstration of actual prejudice arising from the denial of that right. We conclude, therefore, that the violation of G.L.c. 119, § 56, requires the reversal of the defendant's conviction and a new trial.
2. Because the issue may arise again on retrial, we briefly dispose of the defendant's claim that the judge erred in not allowing the defendant to impeach the victim's credibility by reference to the victim's juvenile record. That record contained charges of delinquency by reason of breaking and entering and larceny of a motor vehicle. A judge, in that case, had continued the charges for disposition after finding sufficient facts to warrant a finding of delinquency. Those charges had no apparent relation to the crime for which this defendant was adjudicated delinquent. We shall assume, without deciding, that the judge's action was the equivalent of a finding of delinquency.
General Laws c. 119, § 60, generally bars the use of a juvenile's record in a court proceeding in which the juvenile is a witness. See Commonwealth v. Ferrara, 368 Mass. 182, 185-186 (1975). We have indicated that the provisions of § 60 must yield to countervailing constitutional considerations, such as where the records would be relevant to the issue of bias resulting from the witness's susceptibility to government pressure. Id. See Commonwealth v. Santos, 376 Mass. 920, 925-926 (1978) (use of juvenile records sealed pursuant to G.L.c. 276, § 100B). In another context, where the right to a full defense would otherwise be denied, we have made an exception to the general rule that evidence of prior bad acts may not be used to impeach a witness's *395 credibility. See Commonwealth v. Bohannon, 376 Mass. 90, 93-95 (1978) (evidence that victim had made prior false allegations of crime charged to the defendant).
In this case, the defendant asserts that because the case against him rested on the credibility of the juvenile victim's testimony, the defendant should have been allowed to attack that credibility through use of the victim's juvenile record. We have already rejected such a general contention. Commonwealth v. Santos, supra at 923-924. On the sparse record before us, we discern no special circumstances that would mandate admissibility of the record.
3. The adjudication of delinquency from which the juvenile defendant has appealed is reversed and the case is remanded for a new trial to a jury of twelve persons consistent with this opinion.
So ordered.
NOTES
[1]  The Commonwealth disclaims any reliance on the amendment of G.L.c. 218, § 26, as appearing in St. 1980, c. 122, approved May 2, 1980, by which a violation of G.L.c. 265, § 15A, was placed within the original jurisdiction of the District Courts. Trial of an adult charged with a violation of G.L.c. 265, § 15A, committed after the effective date of the 1980 amendment would not necessarily be on an indictment because a District Court judge could receive and retain jurisdiction over the matter. We think it is clear that a juvenile would not be entitled to a jury of twelve were he charged with a violation of G.L.c. 265, § 15A, committed after the effective date of the 1980 amendment of G.L.c. 218, § 26.

If the Commonwealth had relied on the 1980 amendment, we would be faced with the question whether application of an amendment enacted after the date of the offense would constitute an ex post facto law in violation of State and Federal Constitutions. See the Declaration of Rights of the Massachusetts Constitution, art. 24; United States Constitution, art. 1, § 10. There is a substantial, constitutional question whether the 1980 amendment of § 26 could be applied retrospectively in determining the right to a twelve-person jury under G.L.c. 119, § 56. Compare Thompson v. Utah, 170 U.S. 343, 351-352 (1898), and McSears v. State, 247 Ga. 48 (1981), with State v. Maresca, 173 Conn. 450, 451-454 (1977), and State v. McIntosh, 23 Ariz. App. 246, 248 (1975). See Commonwealth v. Greenberg, 339 Mass. 557, 578-579 (1959); Commonwealth v. Bellino, 320 Mass. 635, 641-642, cert. denied, 330 U.S. 832 (1947).
There would be the initial question, in any event, whether, as a matter of statutory construction "under the influence  if not the command" of ex post facto provisions, we would read the 1980 amendment as prospective only. See Commonwealth v. Davis, 380 Mass. 1, 15-16 (1980). Thus, we accept the Commonwealth's concession that the 1980 amendment is inapplicable in this case.
